Objective To study osteoporosis in patients receiving androgen deprivation therapy (ADT) with prostate cancer, and determine whether once-weekly oral alendronate can prevent bone loss in men receiving ADT. Methods One hundred and twelve men with nonmetastatic prostate cancer receiving ADT were divided into two groups from April 2007 to April 2008, 56 cases in each group. Group A took alendronate (70 rng once-weekly orally) and calcium supplement, group B received calcium supplement only. Bone mineral density (BMD) were measured both before and 6 months, 12 months after treatment for both groups. Results There were no statistically differences in age, persistence time of castration, prostate specific antigen level and adverse effect between two groups(P＞ 0.05). At baseline, 39.3%(44/112) of men had osteoporosis and 51.8%(58/112) had low bone mass. After 12 months treatment, in group A, BMD increased 3.7% (95% CI 2.80% to 4.60% ,P＜0.01 ) at the spine,0.7%(95% CI 0.10% to 1.40% ,P=0.031)at the total hip and 1.6% (95% CI0.40% to 2.80%,P =0.008) at the femoral neck. In group B decreased 1.4% (95% CI-2.70% to -0.03%, P = 0.045 ) at the spine, 0.7% (95% CI - 1.50% to -0.01%,P = 0.052) at the total hip and 0.7% (95% CI -1.50% to 0.10%, P = 0.081 ) at the femoral neck. The estimated changes in BMD were significantly different between two groups (P ＜ 0.01 ). Conclusions It suggests that ADT induce bone loss which should be treated in early stage. Bone loss that occurred with ADT is prevented and improved with once-weekly oral alendronate.