In this study the visbreaking unit of Tehran refinery was simulated and then a parametric sensitivity analysis was carried out for determination of optimum temperature. The Petro-Sim simulator, which specializes in the simulation of refinery processes, was used in this study. Initially the simulator was validated using actual plant test runs and after tuning, the simulations provided errors less than 3%. Using the validated simulator the sensitivity of yield of fuel oil, gasoline and fuel oil viscosity with the variation of furnace temperature (reaction temperature) was investigated. The validated simulator was used to optimize the unit operating conditions to obtain the desired product specifications. The optimum value of fuel oil yield, gasoline yield, viscosity and temperature were 91.51, 6.18, 79.6 cSt and 824 °F, respectively