Background: Intensity Modulated Radiation Therapy (IMRT) technique is an advanced method of radiotherapy leading into the development of Flattening Filter-Free (FFF) medical linear accelerators (Linacs). Monte Carlo simulation has been a standard method for calculation of particle transport due to precise geometry and material specifications. Objective: This study is to obtain the design optimization of Flattening Filter Free (FFF) for 6 MV Linac machine. Material and Methods: In this simulating study, EGSnrc user code was used to simulate particles emitted from head of linac 6MV Varian to achieve the most suitable filter in FFF linac design. Monte Carlo simulation results of the PDD and profile, on the 10 × 10 cm2 field, were compared with the measurements. Differences in small profile beams from Monte Carlo simulation were also evaluated between FF and FFF linac. Results: The spectrum on Monte Carlo simulation in isocenter was compared with Treatment Planning System (TPS) for each filter variation. The slight differences of average spectrum are simulated using 2 mm copper filter and FakeBeam with-1. 52 ± 3. 82% and-3. 13 ± 3. 61%. Whereas, for PDD and profiles, each variation has an average difference of 7. 10 ± 0. 70% and-5. 99 ± 1. 39%. Conclusion: FakeBeam filter is a proper filter for the use of linac design 6MV Varian. It is necessary to decrease the kinetic energy of electrons to perform MC simulations on FFF linac.