Modified radical mastectomy (MRM) is the most common surgery for cancer breast that is associated with marked postoperative pain. Effective control of this pain suppresses the surgical stress response and decreases the opioids and general anesthetics' needs. This study compared the erector spinae plane (ESP) block and the pectoral plane (PECS) block effects on the opioid consumption postoperatively, stress response, fentanyl needs intraoperatively, pain scores, and incidence of complications in female patients subjected to MRM surgery.Fifty patients were allocated randomly and divided into two groups. Forty-seven patients were included in the final analysis after exclusion. ESP block group (E group, = 24) received 20 mL of 0.25% levobupivacaine plus 0.5 μ/kg dexmedetomidine that was injected in-between erector spinae muscle and transverse process. PECS block group ( group, = 23) received 30 mL of 0.25% levobupivacaine plus 0.5 μg/kg dexmedetomidine divided into 10 mL that was injected between the two pectoralis muscles in the interfascial plane and the other was 20 mL injected between the serratus anterior and the pectoralis minor.Postoperative morphine consumption and stress hormone level in P group were significantly lower than E group. The pain scores and number of patients requested analgesic postoperatively showed significantly higher values in E group. Hemodynamic parameters, fentanyl needed intraoperatively and the incidence of postoperative complications recorded no significant difference between the two groups.The current study demonstrated that PECS block provides better quality of analgesia than ESP block in patients subjected to MRM operations.