Fractionated irradiation is one of the important methods studying repair of radiation damage. Some authors reported that caffeine, a repair inhibitor, had no effects on fractionated irradiation in human cell, animal cell and ,Vicia faba., And they concluded that caffeine inhibited the repair of potentially lethal damage(PLD), but did not inhibit the repair of sub-lethal damage (SLD). In this study, the effect of caffeine on the repair of PLD and SLD were reexamined in wheat and barley systems. The germinating seeds of wheat of barley were pre-irradiation treated 3 h with 5 × 10,-3, mol/L caffeine, then they were irradiated by single or fractionated X-ray doses. The results showed that after caffeine treatment the effect of fractionated irradiation was disappeared. In this case, caffeine not only inhibited the repair of PLD but also inhibited the repair SLD.,In previous fracttionated radiation experiments, the repairs during interval were discussed in detail, but the repairs after whole irradiation were neglected. According to the results of this experiment and some early reports about fractionated irradiation, the repair efficiency during interval and the repair function after whole irradiation were discussed.