Background: The effective relief of pain is of paramount importance for patients undergoing surgery. Patient and methods: Sixty patients, aged 35 - 45 years, ASA I and II, scheduled for elective lower limb orthopedic surgery under spinal anesthesia were randomly divided into three equal groups, Control group: were given intrathecal 20 mg (4 mL) of 0.5% hyperbaric bupivacaine. Fentanyl group: were given 17.5 mg (3.5 mL) of 0.5% hyperbaric bupivacaine with 25 μg intrathecal fentanyl. Nalbuphine group: were given 17.5 mg (3.5 mL) of 0.5% hyperbaric bupivacaine with 0.8 mg intrathecal preservative-free nalbuphine hydrochloride. Results: There were no statistically significant differences between the three groups regarding the demographic data or HR. The decrease in MAP was significantly lower in the nalbuphine group when compared with fentanyl and control groups. The onset of complete motor and the sensory block was statistically significant rapid in the fentanyl group when compared with other groups. The duration of the motor and the sensory block was highly statistically significantly prolonged in the nalbuphine group when compared with other groups. Postoperative VAS showed a highly statistically significant difference between the three groups. Conclusion: Nalbuphine (0.8 mg) as intrathecal adjuvants to 0.5% hyperbaric bupivacaine increases the duration of sensory block, motor block, and effective analgesia, and decreases the incidence of intraoperative complication more efficiently than fentanyl in patients scheduled for elective lower limb orthopedic surgery under spinal anesthesia.