Context: Several studies addressing Intrathecal Morphine (ITM) use following spine surgery have been published using low to high-dose of ITM. But the optimal dose of ITM with maximal analgesic property and minimal complication and side effects is yet to be decided. Aims: We aimed to compare the analgesic efficacy of two low doses of ITM, 0.3 mg and 0.4 mg in patients undergoing lumbar spine decompression, with or without instrumentation surgery. Materials and Methods: Fifty patients were enrolled in a double-blinded randomised controlled trial, and received either 0.3 mg or 0.4 mg of ITM. Post-operatively, all patients were given a Patient Controlled Analgesia (PCA) pump and observed for the first 24h in a step-down unit. Measurement of: Total fentanyl used during intra-operative period; Total PCA morphine consumed in first 24h; Intensity of pain; Sedation score; nausea; Vomiting; Pruritus; and occurrence of respiratory depression were recorded. Statistical Analysis Used: Statistical analysis was performed using Graph Pad Prism 6.0, La Jolla, CA, USA. Statistical significance of categorical variables between the groups was compared by Chi-square test and that of quantitative variables were compared using Student’s t-test. Results: The total intra-operative fentanyl consumption and total PCA morphine use and the overall pain score over the first 24 h in the post-operative period was significantly low in 0.4 ITM Group. There was no difference in terms of sedation, nausea, vomiting and pruritus. There was no case of respiratory depression in either Group. Conclusions: 0.4 mg ITM provided superior analgesia in the post-operative period compared to 0.3 mg with no significant increase in the incidence of side effects.