The work is to study the effects of alpha-calciferol on bone mineral density, histomorphometry and biomechanics in rats with osteoporosis induced by irradiation of the rat kidney. 32 male SDrats of six months in age were randomly divided into 4 groups ( 8 rats per group), i.e. the model group,the sham group, the bone one group and the fosamax group. Osteoporosis was developed in the rats by irradiating the kidney. Then the rats were administrated orally as follows in a 90 days, 0.1g•kg,-1, BW.d of alpha-calciferol for the bone one group, 10 mg•kg,-1, BW.d of alendronate sodium in 1mL CMC for the fosamax group, and 1mL CMC for both the model group and sham group. BMD of L1-4, bone gistomorphometry and the bone biomechanical properties were measured. Compared with the model group, both the bone one group and the fosamax group were characterized with sigmificantly higher BMD of L1-4 (,p,<,0.01), significantly larger volume and width of bone trabecula, smaller space of bone trabecula (,p,<,0.05,p,<,0.01), and significantly larger maximal stress of femur and lumbar vertebra (,p,<,0.05,p,<,0.01). It is concluded that Alphacalciferol can improve BMD, bone histomorphometry and bone biomechanical properties in rat osteoporosis induced by kidney irradiation.