Abstarct: In this research, the effect of geometrical shape and dimensions of the 15MeV electron linear accelerator (Linac) on the gamma rays spectrum for production of the neutrons has been studied. We have calculated the range of outgoing electrons from the front of the accelerator within the filter made of stainless steel. Three dominant interactions of generated gamma rays from electron’s bremsstrahlung occur, including Compton scattering, photoelectric effect and pair production, has been studied. We use the method of Monte Carlo simulation using MCNP code. MCNP is a general-purpose Monte Carlo N-Particle code that can be used for neutron, photon, electron, or coupled neutron / photon / electron transport. Geometric space for the filter made of the stainless steel 304 with different shapes and dimensions was simulated using MCNP code. Then the photon flux output from the filter at different angles relative to the direction of the emitted electrons was calculated. The energy distribution of photons produced in different locations was alanyzed for four different shapes: cone, hemisphere, cylinder, and ellipsoid with different dimensions. In comparison between shapes and dimensions optimization in order to produce the maximum photon flux, a cylinder with a height 0.3 cm and radius 0.1 cm has obtained. Also by studying the spatial distribution of produced gamma rays; these rays at angle below 10˚ relative to the direction of the electron emission have the highest percentage of production. Regarding that the spectrum of gamma rays produced by the bremsstrahlung is continuous spectrum, we expected gamma rays with the energy range of 0 – 15 MeV, and can use these gamma rays in (γ, n) reaction which produce neutrons in order to exploit in many works such as in medical centers.