According to The Gazetteer of Sikhim written by HH Rishley and published from Calcutta in 1894 Sanga Chelling (meaning The place of secret spells) Monastery near Pelling build in 1697 is the oldest monastery of Sikkim. While Dubdi (meaning The Hermit's cell) Monastery was build in 1701.
In latter years the Sanga Chelling Monastery got burned and was reconstructed in 1965.
It is also spelled as Sanga Choling Monastery or even as Sangngak Choling Monastery as written in the sign board at the monastery.
pic: from flickr under virgodad's photostream (2007)