Arabidopsis, embryos were irradiated by ,241,Am α-particles to 1, 10 or 100Gy to study the irradiation effects. The results indicated that germination percentages of embryos were not affected by the irradiation in the dose range. Germination speed of the 1Gy group was 11.4% higher than that of control, whereas germination speed of the higher dose groups decreased, with germination speed of the 100Gy group being about 29.3%. The irradiation effect on root length was similar to the germination speed. Moreover, root gravitropism of the 100Gy group became S-shaped curves. In a mechanism study, the root gravitropism recovered while cultured in an MS medium containing 0.2mmol/L Vc, which indicated that H,2,O,2, might joined the process of gravitropism. Furthermore, during shooting period of the ,Arabidopsis, the content of MDA was detected and the activities of SOD and CAT were observed. and the CAT activity and MDA content increased with the dose, while SOD activity of the 100Gy group decreased by 12.6% compared with the control. The results indicate that the induced increase of H,2,O,2, signal molecules after the α-particle irradiation may be the mechanism of radiation physiological effects.