Best Season to Visit : March - May and October - December.
Area :  43 sq km
Distance : 137 km (from Gangtok)
It is located near Yumthang in North Sikkim and contains a vast variety of rhododendrons. The sanctuary is bounded on its southern periphery by the Yakchey La and on the northern periphery by the Yumthang Valley known for its alpine meadow and hot springs.

It is hemmed in on the east by the high rugged Chuba-Sagochen mountain ranges and on the west by part of Chomzomei Tso extending upto Lava pass. The picturesque Yumthang Chhu (river) flows through the sanctuary, which is known for its unique abundance of Rhododendron trees and shrubs, 40 species/varieties of which are recorded from Sikkim alone. When in blossom the rhododendrons provide a riot of colour to this small park.

The fauna here consists of the Serow, Goral, Blood Pheasant , Common Hill Partridge etc. also found here. The beautiful and highly endangered Spotted Linsang or Tiger-Civet (Prionodon pardicolor) has been reported from here.

The rare and endemic Rhododendron nevium, the State Tree occurs only in this sanctuary which is also rich in ground flora such as primulas, potentillas, gentians, saxifrages, poppies and aconites. Brown Trout were introduced into Yumthang Chhu at Phunyi in 1978 and have been flourishing since. Lachung is the last frontier village before reaching the sanctuary. There is a Forest Rest House here, as well as one at Yumthang.