Background: Ultrasound-guided modified pectoral nerve block and erector spinae plane block are simple regional techniques that can be used as perioperative analgesia to decrease the consumption of opioids so decrease their side effects as well as devoid of complications of other regional techniques. Aim: To evaluate and compare between ultrasound-guided modified pectoral nerve block and erector spinae plane block for perioperative analgesia in unilateral breast cancer surgeries. Methods: This prospective, controlled research was conducted in Zagazig University Hospitals on 36 female patients aged between 21 and 60 years, with ASA class (I, II), scheduled for elective unilateral modified radical mastectomy under general anesthesia (GA). Patients were randomly allocated into three equal groups: group C(control group) (n=12):Patients received only GA, group P (n=12):Patients received unilateral ultrasound-guided modified pectoral nerve block before induction of GA and group E (n=12): Patients received unilateral ultrasound-guided erector spinae plane block before induction of GA. Results: There was statistically significant increase in total intraoperative fentanyl and postoperative pethidine consumption in control group compared to group P and E. Visual analogue scale was higher in control group compared to group P and E at different time intervals and higher in group E than group P at 4, 8 and 12 hours but with no significant difference between group P and E 30 min after the operation, 2, 18 & 24 hours postoperatively. There was significant delay in 1st time to rescue analgesia in group P > group E > control group. Conclusion: Ultrasound-guided modified pectoral nerve block provides more duration of analgesia and better pain scores with lesser amount of opioid requirement in comparison with ultrasound-guided erector spinae plane block in the first 24 h after unilateral breast cancer surgeries.