Abstract Background and aims Post-operative analgesia for Spine surgeries is difficult without patient control analgesia (PCA) and inadequate monitoring facilities. The objective was to study the effectiveness of analgesia of intravenous administration of low dose fentanyl and morphine for postoperative analgesia following spine fusion surgeries. Methods One hundred adult patients undergoing spine instrumentation surgeries were randomly allotted into two groups: Group M (morphine) or Group F (fentanyl). The patients received either 0.02 mg.kg-1.h-1 of morphine or 0.3 mcg.kg-1.h-1 of fentanyl infusion postoperatively. If the patient had pain, additional bolus dose of 0.04 mg.kg-1 and 0.6 mcg. kg-1 bolus for morphine and fentanyl respectively were given and noted. The additional analgesic consumption was recorded. The Ramsay sedation score (RSS), visual analogue score (VAS), vital parameters and complications were observed. Results The demographic characteristics did not reveal significant difference among the two groups. In morphine group, 32 patients did not require any additional bolus dose, 15 patients needed one bolus dose and one patient each required two and three boluses. In fentanyl group, two, 24, 20 and four patients required 0, 1, 2 and 3 bolus doses respectively. There were no statistically significant variations in hemodynamic features like heart rate, blood pressure and oxygen saturation, RSS and VAS. The complication rate was not significant among the groups. Conclusion Low dose continuous infusion of morphine is more effective than fentanyl with fewer requirements of rescue analgesics for postoperative analgesia. Both drugs are safe without any serious complications.