Abstract: The sufficient and necessary conditions of baroclinic instability of the basic flow including meridional circulation are K22F and (Usk+Vs1)2U0k2.The baroclinic instability can occur is only meridional flows. If the meridional wavenumber 1 (1>0) is known in westerlies,the direct meridional circulation expands the unstable area and the indirect meridional circulation reduces it oppositely.But the features are contrary in easterlies. The instability growth rate of the basic flow including meridional circuZation varying with wavenumber K is remarkably different from that of only aonaI basic flow.