ir There are different ways for heavy crude oil recovery. One of these methods is a kind of steam flooding named Steam-assisted gravity drainage (SAGD). In an ordinary SAGD process, two parallel horizontal wells are drilled in vicinity of the reservoir foundation. Steam assisted gravity drainage (SAGD) is known as an in-situ heavy oil recovery process, which along with several other auxiliary methods is also known as the most popular method contributing to Original Oil in Place (OOIP) recovery of at least 60%. In this method two well pairs, steam injection and oil production wells are drilled horizontally and parallel in heavy oil reservoir. Hot steam is then injected into through the upper well which includes heat to pour oil toward production well. The steam is injected in order to heat up the oil and reduce its viscosity so that the oil mobility in porous media increases. Because of higher viscosity of oil to the steam, the oil moves down. Thus oil transfer toward the production well is driven by gravity force and the system geometry is such that the oil moves approximately parallel to the interface of steam saturated area (steam chamber) and oil bulk. The target of modeling and studying SAGD is to develop instruments which could enable us to have most capable, cost efficient and applicable mechanism to recover oil from every specified reservoir. Regarding to the previous simulations on heavy oil reservoirs for SAGD by CMG commercial simulator which has a high cost and also sanctions limitations, the need to produce a simulator which could produce reliable results and meets the industrial needs was felt. This study is done to meet our country’s oil industry needs by presenting a model based on analytical elemental model of Butler and solved with initiated numerical method in order to present an exact pattern of steam chamber growth and heat profile in SAGD process. This method is accomplished to set the relations for determining the optimal position of injection and production wells and studying the effects of parameters such as steam injection pressure and temperature. Heat transfer equations ahead of the interface have been written under unsteady state condition. Mass balance equations have also been written and coupled to heat transfer and auxiliary equations in elements in order to obtain temperature, pressure and saturation profiles and more precise steam chamber growth pattern. Alongside with accurate steam chamber position at any time and due to exact mass and heat balance, oil production rate had been calculated accurately. These results can help on optimization of injections and production wells positioning reservoirs. It is seen that the production rate is approximately at the highest point on 2600 seconds after starting injection. Also a pressure of 380 psi is the best pressure for steam injection. Optimal vertical distance is about 6ft and 35ft is the horizontal distance between two well pairs in reservoir calculated by this model. Keywords : Simulation, Steam-assisted gravity drainage, SAGD, vertical distance of well pairs, horizontal distance of well pairs, pressure