Based on the tropopause observation data of eight sounding stations from 1980 to 2004 over Gansu Province, the seasonal variations of the first and the second tropopause heights were analyzed. The results indicated that there are significant seasonal differences in the two kinds of tropopause over Gansu Province. The second (first) tropopause is the main type in summer (winter). The annual average height of the first tropopause is higher in spring and autumn than in winter and summer, the average height of the second tropopause is lower in autumn and winter than in spring and winter. There is an obvious annual variation in the tropopause height in this location, the first tropopause height in each season has a periodic oscillation of 5 to 6 years, the second tropopause heigh has a quasioscillation of 3 years. The two kinds of tropopause heights in each season have a mainly rising trend in Gansu Province in the recent 25 years, especially in summer.