Located at where Taichung County and Yilan County meet， Siyuan Pass is under the administration of Yilan County. 1948 meters in altitude， the pass is not only the lowest saddle in the offshoots of Nanhu Mountain and Taoshan， but also the watershed between the Lanyang River and the Dajia River and a climate watershed. Here is a natural ecological treasure with a warm temperate forest of red cypress and yellow cypress in the upper part and incense nanmu， Formosan michelia， Chinese cryptocarya， yellow chestnut oak， alder and cryptomeria as well as many other animals and fern ecology seen in other areas. Spring and summer with blooming flowers are most suitable for visiting. Autumn and winter with fallen leaves are full of bleak beauty. Sometime， white flakes of snow are falling down when there is adequate moisture.