A computer program based on Monte Carlo method had been designed and written in visual basic computer language and utilized for simulating the classic problem of gamma ray beam incident on finite plane slabs of absorbing materials. The source geometry adopted in this program is plane normal source.Dose buildup factor of gamma photons including annihilation radiation have been calculated in the energy range (4-10) MeV for aluminum, iron, and lead up to 5 mean free path thickness.