In this paper, three dimensional modeling of steam injection in a fractured carbonate reservoir containing heavy-oil was performed. The paper describes a numerical Technique that simulates three-dimensional heat transfer and three-dimensional, three-phase fluid flow in a reservoir.This model considered both capillary and gravity effects in the energy equation and a variable porosity. First, steam injection process was modeled in a non-fractured reservoir, then, the model accuracy was assessed. The results showed that steam injection in a non-fractured reservoir increases the oil recovery to 70 %. In the next step, steam injection process was modeled in a fractured reservoir. The amount of oil recovery in this reservoir was estimated about 30 %.The sensitivity analysis for identification and evaluation of steam injection process in both non-fractured and fractured reservoirs was performed. The results indicated that some parameters such as: injection pressure, steam quality, pre-heating, permeability and location of fracture effect on the oil recovery. Oil recovery from carbonate fractured reservoirs is less compared with non-fractured reservoirs, and breakthrough occurs sooner.