We designed this study to determine the optimal intrathecal dose of morphine in patient undergoing hip or knee surgery. The optimal intathecal dose was defined as the dose that provided effective analgesia with minimal side effects for 24h after surgery. Sixty patients, scheduled for hip or knee surgery, were randomly allocated into 3 double-blinded groups. Group 1 and group 2 received intrathecal morphine 0.3 and 0.5 mg respectively while group 3 received normal saline (NSS) 0.5 ml. All were given a standardized bupivacaine dose intrathecally. Pain scores, IV pethidine injection, and morphine-related side effects (respiratory depression, postoperative nausea and vomiting, pruritus, urinary retention and sedation) were recorded for 24h after surgery. VAS score recorded postoperatively at 6 and 12 h in both morphine groups were significantly less than NSS group. Time to first analgesic requirement was significantly longer in both morphine groups compared to NSS group (group 1=15.1 ± 6 .7 h, group 2 = 14.7 ± 6.8 h, group 3 = 8.6 ± 5.4 h, p value = 0.0001). The number of additional pethidine injection reguired in the first 24 hours was significantly less in the morphine groups (2.22, 2.82 and 4.83 in group 1, 2 and 3 respectively, p value = 0.0001 for group 1 and versus group 3). Incidence of postoperative nausea and vomiting (PONV), pruritus and urinary retention were significantly higher in both morphine groups compared to NSS group. Respiratory depression did not occur in all patients. We conclude that 0.3 mg of intrathecal morphine in the optimal dose for pain relief after hip of knee surgery with acceptable side effects.