OBJECTIVE To study the radioprotective effect of orally administered Diltiazem in phase Ⅱ clinicaltrial METHODS The model of tumor patient radiated at the latter half of body was used. Clinical symptom, the count of WBC and PLT and radio of lymphocyte abnormal chromosome of patient were observed RESULTS Investigation on 22 patients receiving radiotherapy showed that Diltiazem reduced the side clinical symptom caused by irradiation. At day 1 through day 21, the counts of WBC and PLT of Diltiazem group had various degree of increase than irradiation and the difference between Diltiazem group and control group was significant at some time Dicentric yield of the Diltiazem group was lower than that of the control. According to the dicentric yield the estimated average absorbed dose of the total blood of the untreated patients was 1.52 Gy, Whereas that of the Diltiazem administered group was only 0.86 Gy CONCLUSION It can be assumed that Diltiazem can reduce the side effect of the tumor patient caused by irradiation under these experimental conditions.