Abstract Background: Cancer breast is the most common cancer in women and 2nd most common cancer overall. Post-Mastectomy Pain Syndrome (PMPS) is a term used to describe chronic pain following both partial mastectomy or mastectomy. It is one of many post-surgical persistent pain syndromes. Studies estimate that PMPS develops in about 25% to 60% of women leading to disability, psychological distress, and diminished quality of life. Aim of Study: The aim of the study was to compare analgesic efficacy and duration of stellate ganglion block when using both dexamethasone and magnesium sulfate with the local anesthetic lidocaine in comparison to using dexam-ethasone alone with lidocaine in cases of post mastectomy pain syndrome. Patients and Methods: Current double-blind study was conducted from September 2019 to September 2020 at outpa-tient pain clinic in Oncology Center Mansoura University (OCMU). Fifty female patients between age of 20 and 60 years, ASA physical status I and II underwent breast surgery. The patients were randomly assigned by computer generated sequence of random numbers method according to the drugs used into 2 equal groups, each group consisted of 25 patients, one group had received Dexamethasone and Lidocaine (group D) and the other group had received Magnesium, Dexameth-asone, and Lidocaine (group DM). After block, pain score was obtained using Visual Analog Scale (VAS). The shoulder movements were measured using goniometry. The skin tem-perature of the ipsilateral arm to the block was also measured. Results: After block, pain score was obtained using Visual Analog Scale (VAS). The shoulder movements were measured using goniometry. The skin temperature of the ipsilateral arm to the block was measured, the different types of shoulder movements were significantly better after injection of SGB in both groups compared to before injection. DM group showed longer analgesic duration and lower VAS score with less analgesia needed compared to D group. Conclusion: Adding magnesium sulfate to lidocaine and dexamethasone for ultrasound guided stellate ganglion block in cases of post mastectomy pain syndrome increases the time to first request of analgesia, decreases VAS values and the total amount of analgesia needed in the follow-up.