Abstract: The southern side of the Nan Ling Mountains in South China gives rise to an east -west aligned stationary front that produces much rain in May -June. Its symmetric instability is discussed. According to Hoskins (1974), it won't occur in zonal flow unless geopotential vorticity turns negative, with friction and non-adiabatic heating as the necessary condition for its generation.It is argued that in a basic flow unstable or mildly stable stractification facilitates unstable development of a disturbance if absolute vorticity is positive; stable stractification, however, can be a favourable factor when the vorticity becomes negative. Wave-CISK mechanisms and stractification are conditioned each other in their common effects on a disturbance. Regarding the instability due to forced lifting by relief, it is shown that in stable stractification polewardly decreasing pattern of θse isopleths favours disturbance development in a front over the northward tilted slope while the reverse is true over the southward tilted slope.