To characterize factors determining the varietal difference of the fruit size in sweet cherries, we studied the relationships among fruit weight, number of mesocarp cells and radial cell lengths in 56 cultivars and selections of sweet cherries and related species. In addition, the effect of flower thinning on the number of mesocarp cells and the dependence of cell number on young fruit weight were analyzed. A high correlation coefficient was obtained between fruit thickness and mesocarp thickness versus fruit weights. All parameters measured showed large varietal differences. The correlation coefficient between mesocarp cell number and length to the differences in fruit weight varied yearly from 0.611 to 0.706, and from 0.488 to 0.599, respectively. The contribution of the cell number to fruit weight was larger than that of cell length in every year tested. The yearly correlation coefficients were 0.869-0.931 for the fruit weight, 0.826- 0.852 for the number of mesocarp cells, and 0.482-0.674 for the radial cell length. Fruit weights of the wild species were considerably lighter than those of the commercial cultivars. The numbers of mesocarp cells and cell lengths of the former were about half those of the latter. From these results, we conclude that both the mesocarp cell numbers and their lengths affect the varietal differences in the fruit weight. However, the cell number has a larger contribution and is a more stable parameter. Young fruit size and flower thinning had almost no effect on mesocarp cell number. Therefore, we conclude that cell number determination in mesocarp is a useful method for estimating varietal differences in fruit weight of sweet cherry.