A three-dimensional primitive equation model on a sphere is constructed to investigate stability of a baroclinic flow in cyclostrophic balance and structures of the unstable modes. When the Richardson number is large, the baroclinic instability appears for sufficiently small values of the Rossby number. As the Rossby number increases, only the large scale modes remain unstable and amplitude of these modes becomes confined to the lower layer. When the Richardson number is small, the symmetric instability predominates, while the baroclinic instability modes also exist in a certain parameter range. Unlike the baroclinic instability mode, the amplitude of the symmetric instability mode is confined to low latitudes in the upper layer. Since the Richardson number is small in the Venus cloud layer, it is predicted that the baroclinic instability is suppressed in the midlatitudes, and the symmetric instability appears in the low latitudes.