To clear whether there is any relation between spikelet number and other quantities of the plant，dry matter production or leaf area, is important to make practical mechanistic model for assuming spikelet number.
Rice plants of two growing seasons, normal and late transplanting, were grown under two thinning and three shading conditions during the whole vegetative stage or during the panicle formation stage. Spikelet number could be indicated by one equation which contains three parameters, dry matter production for vegetative stage, for panicle formation stage and leaf area on the end of vegetative stage. Calculated number of spikelet by the equation relatively fitted the actual that on three year data.
We suggested that the equation is possible to use for assumption of spikelet number if other model is used to assume the dry matter production and the leaf area from meteorological data.