Abstract: In order to study the mechanisms of yield increase in hybrid rice, we have observed and compared the morphological structure on flag leaf surface in Wild-abortion type hybrid rice Shah-you No. 50 and its Three lines under scanning electron microscopy. The results are as follows: The morphological structure of flag leaf surface of hybrid rice is similar to its Three lines form, but exist differences of some structure, usual deviating to male parent on growing and looking of hybrid. The length of all observed samples is 11.4—17.4μm, among which mean length of restorer is 12.2μm and is the shortest; the mean length of hybrid rice is 15.5μm and is longest. The surface area of stomatal complexes is 164.3—243.9μm², among which mean area of stomatal complexes of restorer is the smallest i. e. 171.91μm²,that-of hybrid rice is the largest i. e. 222.1μm². The difference between maintainer and restorer is not significant, but those between maintainer and malesterile line is significant as well as those between maintainer and hybrid rice is very significant. According to these results, as compared with the yield, we deduce that the increasing yield is related to enlarged stomata in hybrid rice.