Dramatic increases in fruit size have accompanied the domestication of Pyrus pyrifolia. To evaluate the contribution of cell division and cell enlargement in the evolution of fruit size, the following study was conducted.Three wild Pyrus and 46 cultivated Pyrus pyrifolia cultivars were selected to examine cell number/size at time of pollination and at time of fruit harvest. The period of cell division was estimated by logarithmic curve of the increasing pattern of cell number, and its correlations with maturation period and final fruit size were analysed.Final fruit size is directly related to the number of cells produced in the period immediately following pollination. Late-maturing cultivars are larger than earlier-maturing cultivars and this is due to an extended period of cell division.The evolution of fruit size in P. pyrifolia has mainly resulted from shifts in the ability of cells to divide rather than to enlarge.