Erector spinae plane block (ESPB) is commonly done under ultrasound guidance (US) but not all physicians have US and not all of them well trained on it. Therefore, the aim of the work is to compare the analgesic efficiency of both blind and US of thoracic ESPB in post mastectomy pain control. Methods: This prospective, randomized, double-blinded was conducted on 90 female patients between 20 and 70 years of age and ASA physical status (I & II) scheduled for elective breast cancer surgery. Patients were randomly allocated into two equal groups, Group B received blind ESPB and Group U received US guided ESPB. Results: There was no statistically significant difference between both groups regarding VAS, morphine consumption, heart rate, mean arterial pressure, postoperative nausea and vomiting, block performance, ease of performance and duration of surgery. Duration of anaesthesia was statistically significant lower in the blind group than US group. Satisfaction was statistically significant difference between both groups. Conclusions: Blind ESPB could be done safely and easily, and it was not be inferior to US guided ESPB