Modeling and simulation of furnace H-202 in Tehran oil Refinery Company was performed by Fluent software to optimize consumed fuel, and enhance environmental protection commitments. The furnace is natural draft and it has equipped with four burners at its bottom. Simulation of furnace structure was performed by Solid Work software and meshed by Gambit software. Due to symmetrical structure of the furnace, it was simulated in 3D mode. All simulations were performed under steady state condition. Pressure drop in natural draft furnace is very important thus all the convection and radiation tubes are simulated in this simulation. The structure of the furnace is complicated, in this regard to optimize the volume of the calculations the optimum mesh size of 1700000 was used in this simulation.Initially, after validation of the developed model, the result of model was used for estimating the optimum condition for operation the furnace. In this work, the effect of variation of excess air as important and effective parameters on efficiency of the furnace was studied through simulation of the furnace temperature and regarding combustion state. The result was used for evaluation of its optimum value. Simulation results showed that natural draft was an effective parameter on thermal efficiency of the furnace’s convective section. By reduction of the draft, velocity of combustion gases was decreased, and consequently convective heat transfer coefficient decreased, the results of velocity calculation in this section shows that the velocity of 3.5 m/s is the best velocity for reaching to the maximum efficiency in convection section.The results shows that the furnace in 75 Pa draft pressure has optimum efficiency in both convection and radiation sections. Equilibrium mechanism is an ordinary mechanism for modeling of combustion although including equilibrium assumption, the differences between experimental data and simulation result was reasonable. For further study on combustion modeling another, mechanism of GRI-Mech 3.0 was also investigated for combustion modeling. The differences between the gained results and experimental ones are reasonable. The error between the simulation and experimental results for temperature at entrance of convection zoon is 7.1 percent for equilibrium mechanism, and is 11.1 percent for GRI-Mech 3.0 mechanism. The error between the simulation and experimental results for CO 2 concentration in entrance of convection zoon for equilibrium mechanism is 30 percent and for GRI-Mech 3.0 mechanism is 20 percent. One of the reasons for these errors is leakage of air from the existing connections in convection parts. Another reason is the modeling assumptions. The GRI-Mech 3.0 mechanism has the ability to predict the amount of NO which is very important in environmental protection.