Abstract: A three-dimensional axial-symmetric model of cumulus development is here ptesented,with consideration of the effect of condensation feedback on the field of motion.A computational scheme of the system of equations concerned is given.For a case with initial temperature disturbance 1℃ and the vertical lapse rate of circumstances is slightly decreased with height,a calculation computed by the electric computer shows that a cumulus with a thickness more than 5-6 kms could be developed in about 20 minutes In the lower part of the cloud there is a moist-unstable region (a>0) which becomes quasi-steady in its intensity and size.The field of motion and that of temperature are also approximately steady in this region.But in the upper moist-stable region of the cloud,the flow and temperature fluctuate gxeaGly;in this region the connective element from the lower part of the cloud oscillates and gradually dissipates.All these characteristics are in accord with the phenomena observed in the atmosphere,i.e.the cumulus has a steady and flat base but an ever-changing top.