Abstract: In this paper, based upon the conditions of vapour and vertical motion in the precipitation process, a formula for calculating precipitation efficiency was derived. The formula was used to calculate the precipitation efficiency and the precipitation stimulation possibility of frontal clouds du-ring the first rainy season in South China. The results showed that the precipitation efficiency in natural state is lower (about 54.58% in average), However, it is increased to abo.ut 64.71 % with seeding, with a relative increase of 17.42% (α<0.05). Therefore, it is feasible to enhance precipitation efficiency and to realize precipitation enhancement on the ground by seeding.