The largest monastery and one of the Ulaanbaatar’s most impressive sights, Gandantegchilen Monastery was founded in 1809 under the name of “Shar sum” at a place called Dalkhiin Denj. Monastery’s construction establishment started in 1838 by the fifth Bogd Gegeen. The meaning of the Gandantegchilen is "complete rejoicing".

The second Jebtsundamba Khutukt  established a tsanid (Buddhist education center) for the monks of Khüree (modern Ulaanbaatar) in the early 18th century. Datsan (colleges) were built at Dashchoinpel (in 1736), Gungaachoilin (in 1809), and Idgaachoinzinlin (in 1912).

At Gungaachoilin, on a hill west of Khüree, Shar Sum (Yellow Temple) was built in 1809, followed by the Lamrin dugan (assembly hall) in 1824. The fifth Jebtsundamba Khutuktu built a palace, named Gandantegchinlen, just south of Shar Temple between 1834 and 1838; the monastery itself also took this name. Other buildings were added, including the Tsogchin dugan in 1839 and Ochirdari (Vajradhara) Temple in 1840, which has a silver and gold statue created in 1683 by Undur Gegeen Zanabazar. Jud (Tantra) Temple, founded in 1739 by the second Jebtsundamba Khutuktu, is now housed in a Ger. The fifth, seventh, and eighth Jebtsundamba Khutuktus were interred at Gandan, though in 1855 they and their monks moved back to their original palace near the current city center. 

Religious purges of 1937:

Most monasteries in Mongolia, the purges of 1937 fell heavily on Gandan. Gandan was closed in 1939 during the anti-religious persecutions in Mongolia, and its dugan buildings were razed. Escaping the mass destruction of other monasteries, its surviving buildings were used to house the administration of Töv Province. In 1940, the monastery was placed under direct Soviet military command and used as a stable, ammunition store, and firing range. From 1944 to 1989, it was the country's only active monastery as a ‘show monastery’ for foreign visitors. In 1990 its full religious ceremonies recommenced. Today, it is the center of Buddhism in Mongolia and more than 600 monks belong to the monastery.

Megzed Janraisag Temple:

The monastery’s main attraction is Megzed Janraisag Temple. During the religious purge of 1937, more than 700 monasteries and temples were destroyed. 6 temples have been preserved from the Gandantechenlin monastery complex. One of which is the temple of Megzed Janraisig which was the tallest building in Ulaanbaatar until the 1940s.

Lining the walls of the temple are hundreds of images of Ayush, the Buddha of Longevity, which stare through the gloom to the magnificent Migjid Janraisig statue. The original statue was commissioned by 8^th Jebtsundamba Khutugt.

To celebrate independence from Qing Dynasty in 1911, the eighth Jebtsundamba Khutuktu ordered the construction of Megzed Janraiseg (Eye-Opening Avalokitesvara) Temple, completed in 1913 as Mongolia's tallest monastery building. It had a 25.6-metre (84 ft) statue of the Boddhisatva Avalokiteśvara made of gilded bronze and precious stones. The statue of Avalokiteśvara was broken apart in 1937 (alternatively by Soviet soldiers in 1938 or 1943), and is thought to have been shipped to Moscow and melted down.

The new statue was dedicated in 1996 and built with donations and offerings from Mongolians. 20 tons of Erdenet copper castings and 27 tons of steel were used to rebuild the 26.5m high Megzed Janraisig. With a gilt gold covering, the hollow statue contains 27 tonnes of medicinal herbs, 334 Sutras, two million bundles of mantras, plus an entire ger with furniture.

Buddhist beliefs about Avalokitesvara Megzed Janraisag:

In the Avatamsaka Sutra, a Mahayana scripture that precedes the Lotus Sutra, described him as a compassionate bodhisattva who hears the cries of sentient beings and who works tirelessly to help those who call upon his name. A total of 33 different manifestations of Avalokiteśvara are described, including female manifestations, all to suit the minds of various beings

One prominent Buddhist story tells of Avalokiteśvara vowing never to rest until he had freed all sentient beings from samsara. Despite his strenuous effort, he realizes that many unhappy beings were yet to be saved. After struggling to comprehend the needs of so many, his head splits into eleven pieces. Amitaba, seeing his plight, gives him eleven heads with which to hear the cries of the suffering. Upon hearing these cries and comprehending them, Avalokiteśvara tries to reach out to all those who needed aid, but found that his two arms shattered into pieces. Once more, Amitābha comes to his aid and invests him with a thousand arms with which to aid the suffering multitudes.

Avalokiteśvara’s central mantra is the six-syllable “om mani padme hum”, also called the Maani mantra.