Abstract: In this paper, the evolution of individual cumulus is studied with basic equations involving the drag effect of drops and phase transformation of water(including the evaporation caused by entrainment). With individual cumulus being taken as a uniform cloudy parcel,.calculations are made for the thermal conditions and other properties in the prosess of evolution. It is shown that, the effects of drag and phase transformation on cloud development are important. The results are discussed and compared with the observations made in this country, and fair agreement is found in many aspects.