Analgesic effect of low does intrathecal morphine was evaluated in thirty patients underwent transurethral resection of the prostate gland. By prospective randomized double blind control study, each group of ten patients received either placebo (control) or intrathecal morphine 0.1 or 0.5 mg with bupivacaine spinal anesthesia. Visual Analogue Scale and side effects were evaluated at approximately 6, 24, 36 and 48 hr after intrathecal injection. The results showed that patients received intrathecal morphine had significantly lower Visual Analogue Scale comparing to the control group at 6 and 24 hr after intrathecal injection (p<0.05). There was no significant difference in antianalgesic effect (VAS) between patients received 0.1 and 0.15 mg morphine at 6 and 24 hr after intrathecal injection. No significant difference in incidence of pruritus, nausea, vomiting and sedation between the control and treatment groups. None of the patient in all groups developed respiratory depression. The result indicates that 0.1 mg of intrathecal morphine is the optimal does for postoperative pain relief for transurethral prostatectomy.