Background: A large number of patients undergoing major surgical procedures for the management of breast cancer complain of acute postoperative pain. The present study aimed to evaluate the effect of dexmedetomidine when added as an adjuvant to bupivacaine on the quality of this block after modified radical mastectomy (MRM) surgery. Patients and methods: Patients were randomly divided into two equal groups after induction of general anaesthesia. Bupivacaine group (I) (n = 30): received bupivacaine 0.25% in a dose of 0.5 ml/kg injected superficial to serratus anterior muscle. Bupivacaine–dexmedetomidine group (II) (n = 30): received bupivacaine of 0.25% as described above plus dexmedetomidine (0.5 μg/kg) injected superficial to serratus anterior muscle. Results: Requesttime for first analgesic (duration of analgesia in group II (19 ± 3 h) when compared to the group I (14 ± 4 h) and decrease doses of morphine consumption postoperatively (group II (6 ± 2 mg/24 h) when compared to the group I (10 ± 2 mg/24 h)(p 0.01). Conclusion: Dexmedetomidine used as an adjuvant to bupivacaine in ultrasound-guided serratus plane block during modified radical mastectomy surgery leads to an increase of analgesic effect and decrease doses of morphine consumption postoperatively.