Cotyledon cell number and cotyledon cell volume are two main components determining seed size (Mathew et al., 2000). Our experiment suggested that reduction of cotyledon cell number is main reason for seed size decreased induced by UV-B radiation.UV-B radiation decreased cotyledon cell number of three soybeancultivars by 12.3% in our experiment. Since there were no significant differences in cotyledon cell volume, cell growth rate and cell weight, we propose that only the difference in cotyledon cell number, caused by UV-B radiation, was responsible for the observed variations in seed size within the three genotypes tested. However,Hirshfield et al. (1992) reported that seed size may be influencedmore by cotyledon cell size than by cell number. There was apossible reason that different soybean cultivars showed differentcotyledon cell sensitivity to environment change. It seems clearthat the effects of cotyledon cell number on seed growth rate cannot be separated from possible effects of assimilate supply during seed growth period