【Abstract】 Objective To evaluate the effects of intra-articular different dose hydromorphone injection for postoperative analgesia after knee arthroscopy. Methods One hundred-fifty patients scheduled for knee arthroscopy were randomly divided into five groups with 30 patients in each group, according to random number table. The group H1was given hydromorphone 0.1mg (make the volume to 10ml with saline) intra-articular, and group H2 was 0.2mg, group H3 was 0.3mg, group M was given morphine 2mg, group C was given 10ml saline intra-articular. Analgesic efficacy was evaluated by visual analogue scale (VAS) 4, 6, 8, 12 and 24h after administration of different dose hydromorphone, morphine or placebo. The number of patients needing postoperative analgesic and the side effects were evaluated. Results The VAS scores of 24 hours, group H1, group H2, group H3 and group M were significantly lower than group C(P＜0.01). Group H1, group H2, group H3 and group M were no significantly difference at 8 hour after operation. The VAS scores , (2.5±0.6) points in group H2, (2.1±0.7) points in group H3 and（2.3±0.8）points in group M were significantly lower than (5.6±0.6) points in group H1(P＜0.01) at 12 hour after operation, and (2.2±0.5) points in group H3 was lower than (3.1±0.8) points in group H2 and (3.0±0.6) points in group M（P＜0.05）at 24 hour after operation. The number of patients needing postoperative analgesic was decreased with the dose of hydromorphone increased. Conclusion Compared with placebo, different dose intra-articular hydromorphone and morphine can provid a good analgesic effect after arthroscopic knee surgery, and 0.3mg hydromorphone was the best.