Abstract: The heat island circulation induced in unstable stratification is investigated by using thermodynamic and dynamic equations of atmosphere in log-pressure coordinate. The basic features of heat island such as spatial structure of vertical motion, horizontal motion and temperature are given. The results testify in theory the possibility that heat island circulation also occurs in unstable urban boundary layer. The results show that (1) ascending motion is in the urban, sinking motion in the suburbs, a closed center in the lower layer (z=150m), (2) the flow field appears to be a wave over the urban, the wave length is four times the half width of city, (3) the high temperature zone is in the downwind of the urban, and there is no any inversion layer