Brunt-Vaisala frequency squared (N2) measures the static stability of the atmosphere, and reflects the general structure of the atmosphere in term of vertical temperature gradient. For middle atmosphere the response of the middle atmospheric structure to the global warming still lacks investigation currently. The historical data from rocket sounding network in 1962-1991 are used to investigate the long-term trend of N2 in the middle atmosphere. For six stations spanning from the tropical latitudes to the northern mid-latitudes, our estimates show that, in the upper stratosphere and middle mesosphere, i. e., 48-60 km high, the significant decreasing of static stability is observed in an N2 anomalies averaged over 48-60 km range. For two tropical stations, long-term trend in N2 exhibits a similar magnitude, i.e., -0.11×10-4 s-2/decade; it is also observed that the trend increases with latitude, with trend estimates from -0.16×10-4 s-2/decade at 22°N (Barking Sand station) to -0.22×10-4 s-2/decade at 38°N (Wallops Island station).