---

Efficiency analysis plays an important role in price regulation in the electricity distribution sector. This paper analyses efficiency and productivity of 38 electric distribution companies in Iran from year 1387 to 1389 (Iranian calendar year) by using slack based model (SBM). Super efficiency analysis is employed to rank full efficient companies. According to results, Tabriz, Ahwaz and north Khorasan companies have best performance among others. To examine importance of losses inclusion as input on super efficiency scores, statistical tests are utilized. Results indicate significant difference in super efficiency scores with and without accounting for losses. Average productivity index of total companies has declined by 4 percent under investigated period. Further, Panel data analysis applied to specify determinants of super efficiency of electricity distribution companies. According to results, Loss rate, network density and transformer load factor are the main determinants of super efficiency.