Complex regional pain syndrome (CRPS) is a variety of painful conditions following injury which appears regionally having a distal predominance of abnormal findings. This study, evaluate the use of phentolamine for sympathetic block and regional anesthesia in treatment of CRPS related pain. In this study, 68 patients with CRPS who were referred to pain clinics of Imam Hossein and Akhtar Hospitals and Gandy Center of Surgery between 2003-2008 were evaluated. Forty three of 87 patients finally undertaken intravenous regional sympatholytic block according to therapeutic protocol. 37 patients (86%) received one block, 2 of them (4.75%) received 2 repetitions of blocks and finally repeated block for three times occurred in 4 patients (9.3%). A week after block pain relief outcomes was recorded as following; excellent in 7 patients (16.3%), good in the 32 patients (74.4%) and moderate in the 4 patients (9.3%). After a month, 8 patients (18.5%) showed excellent relief and it was good and moderate in 32 (78%) and one case (2.4%), respectively. Pain relief after three months was excellent, good and moderate in the 13 patients (31.7%), 25 patients (61%) and 3 patients (7.3%), respectively. In this study level of pain relief was significant in various intervals and it showed significant difference in relief three months after block (P=0.04). CRPS due to SMP(sympathetically maintained pain) is thought to be alleviated by phentolamine. Intravenous phentolamine infusion is potentially a new significant option for the therapy of CRPS.