Abstract: A comparative observation on the rice pollen callus cultured on the medium supplemented with 2 mg/L 2,4-D and omitted 2,4-D but contained 0.5 mg/L kinetin and 0.2 mg/L NAA respectively was made by electron microscope. The callus cells when transferred to the medium containing kinetin show some changes at ultrastructural level. The numbers of mitochondria, plast and ribosome show an increase in some epidermal cells which kept an ability of active division and may be differentiated into a primordial bud. The storage materials such as lipid bodies and starch grains show sharp decrease or disappearance and the degree of vacuolation decrease in the parenchyma cells of internal callus. Besides vessel differentiation, some sieve elements appeared in the callus. The types of callus cell became various and some electron-dense cell occurred. Although above-mentioned ultrastructural changes appeared in callus cells grown under differentiation condition were true but we do not think those are specific mark for differentiating cell. More work is needed with cytochemistry and molecular biological methods to study callus cell differentiation.