Technetium99 is an important radioisotope in industry, Agriculture and Nuclear Medicine so that more than 75% of producing different types of medical radioisotopes with the use of diagnostic imaging is allocated to this radioisotope. A mono energy ray 140keVemitted from this radioisotope is suitable for imaging. The most commo way of producing this radioisotope is to gain . Molybdenum99 emits through an isomeric process and decay to . The aim of this research is to produce y an electron linac. In this research linac with the energy of 510 MeV and current of electron per second considered to produce neutrons with proper energy and flux according to Bremsstrhlung radiation. The optimum characeteristics of the target to produce the most amount of technetium99 is determined by simulation of the process in MCNPX mont carlo code. At first thick Uranium target with the radius of 1cm and thickness of 4cm was placed in the distance of 1mm from the source of electron. A pod was placed all around the uranium target in distance of 0.5cm. The energetic electron produce Bremsstrhlung photons in the thick target, then this photons produce neutron in the same target. This photo neutrons is obsorbed inod and withray are prodused , . The optimum Radius and thickness of pod around Uranium is calculated by proper Tmesh in MCNPX code regarding the maximum neutron absorption in . The proper thickness were gained in the range of 1 to 1.5cm. After that Tungsten target was used instead of Uranium and optimum conditions were compared with the previous stage. Finally the production rate of Molybdenum99 in optimum conditions in different linac energies was investigated. It was shown that roduction rate has a linear relation with energy of electron beam.