Abstract: By using the concept of moist potential vorticity, the characteristics of moist potential vorticity are analyzed in several hail events over southern Yunnan on March, 1997. The results show that very stiff moist isentropes could lead to the development of moist slantwise vorticity and the initiation of hails. The lower atmosphere has convective stability (MPV1 > 0) in hail area, but the development of moist slantwise vorticity (MPV2 < 0) could be a favorable condition to the severe convection there. Moreover, it is a good environment for hail weather to develop on the north side of areas where MPV2 > 0 and the south side of areas where MPV2 < 0.