Using the random numbers theory performed by MCNPX code, a standard pressurized water reactor assembly loaded with uranium oxide of enrichment 2.1% was modeled. The main goal of this paper is studying some important parameters which participate significantly in the safety operation of light water reactors. These parameters included the neutron multiplication factor, the flux and the power distributions. The variation of uranium and plutonium isotopes with burnup was also studied. The previous parameters were estimated at the beginning of reactor cycle and at different stages of burnup. Then some obtained results were compared with other published results. A good agreement is found between the two results. From the analysis of these results, it was found that the normalized power and normalized flux values vary according to the locations of fuel rods in the assembly. The results also showed the buildup of plutonium isotopes and the consumption of Uranium-235.