Simulation of a reactor core is usually performed using computer codes. Using these simulation codes, design parameters are obtained at a short computation CPU time. This method of calculations optimizes economics and consumption time. In this thesis, a VVER-1000 reactor core has been simulated using the MCNP code. We have obtained the effects of control rod groups (9&10) on the neutron flux and reactivity. Taking minimum power level, the critical concentration of boric acid (H3BO3) approximately equals to 7.55 gr/kg at temperature of T=280 c.