Technetium 99m is the most widely used radioisotope in the world, that it is being used for more than 80%of nuclear medicine procedures. This radioisotope is produced directly from the cyclotron accelerator, but its short half-life of 6 hours makes it impossible to send it for longer distance areas. A good substitution for technetium 99m is to use the molybdenum 99 radioisotope, because it converts to technetium 99m with half life of 66 hours, allows it to be sent to longer distance areas. Most of these radioisotopes are produced by reactors, but it is also possible through accelerators, including linear electron accelerators. In this project, with MCNPX code, the simulation of 99 molybdenum production through linear electron accelerators of 10 and 18 MeV was investigated. Finally the optimum geometry of the photoneutron target was found and the activity of molybdenum 99 was obtained through a linear accelerator of 18 MeV in the amount of 469 mci at the end of 48 hours.