Long term reactivity control comprises the compensation of the reactivity effects of fuel burn up in the PWR reactors. An elegant way for such control is the use of burnable poison (BP) in fuel elements. The use of boric acid in the moderator, B 4 C-Al 2 O 3 rods as fixed burnable poison, boric coated UO 2 fuel, and Gd 2 O 3 -UO 2 fuels are the main methods for the insertion of burnable poisons into the reactor core. The use of burnable poison rods instead of soluble poisons gives another advantage. In PWRs, high boron contents in the moderator gives a positive moderator temperature coefficient. By mainly controlling the reactor by means of burnable poisons, the boron content is reduced and positive moderator temperature coefficient avoided.In this research, we investigated the burnup characteristics of a fuel assembly by varying the concentration of burnable poison in a fuel rod, and the number of burnable poison rods in all assemblies. Furthermore, examined the burnable poison distribution pattern and the influence of changing poison material on the main neutronic parameters of the reactor core. The performance of chromium diboride and gadolinium oxide was also analyzed in this study. The VVER-1000 reactor was used as our test case. Results showed that burnable poison rods reduce the power around burnable poison rod locally so it increases global ppf. Thus the power distribution is far from flat station. By increasing in content of poison and number of burnable poison rods, effective multiplication factor will be reduced at the beginning of cycle. Cases that have equal number of BPRs, but have different layout, and cases that have different number of BPRs, are compared in this research too. With the increase in number of BPRs, effective multiplication factor has reduced at beginning of cycle. Considering the curves for two different absorber (CrB 2 + Al or Gd 2 O 3 ) concentration, the differences in reactivity are larger at beginning of the cycle, then gradually decreases, and at about 5 GWD/TU (for the cases with CrB 2 + Al), the curves become almost parallel. The gained results may provide useful information about the capability of a poison configuration to control the assembly reactivity, and they are particularly useful in order to compare the performance of a different burnable poison distribution.