Background Thoracic paravertebral block may be used for analgesia after breast surgery. Ultrasound can be used during the whole technique of paravertebral block to increase success rate and decrease its complications. As well, pectoral nerve block is now used for pain relief after modified radical mastectomy with or without axillary clearance. Objective To compare thoracic paravertebral block and pectoral nerve block for postoperative analgesia after modified radical mastectomy Methods The study was performed over 30 female patients that were randomly divided into 2 groups with 15 patients in group A for thoracic paravertebral block (TPVB) and 15 in group B for pectoral nerve block (PECS) with injection of total 20 ml bupivacaine 0.25% in each block. Outcome measures of the study are postoperative analgesia duration (time to first rescue analgesia (0.5 mg/kg pethidine) after administration of block) and total analgesic dose in 24 h after surgery and postoperative pain which will be assessed using a visual analog scale (VAS, 0–10 as 0 = no pain and 10 = worst imaginable pain). The vital signs and pain score will be recorded at 0, 1, 2, 4, 6, 8, 12, 18, and 24 h after surgery. Results Our study showed decrease in systolic blood in PVB group immediately postoperative and in the first 6 h postoperative with p value < 0.05. Less time to perform the block in PECS group with p value < 0.001. Less VAS score in PECS group with statistically significant difference between groups at 1 h, 2 h, and 4 h. More time is needed for the 1st requested rescue analgesia in PECS group with p value < 0.05. Patients in the PECS group received less total dose of pethidine with a p value < 0.05 Conclusion The PECS can be effectively and safely used, provides better relief of pain and less hemodynamic changes compared with the TPVB, and reduces postoperative analgesic consumption. Therefore, the PECS can be used safely for postoperative analgesia in patients undergoing breast surgeries with axillary dissection.