The pres ent work stud ies the effect of in troducing MOX fuel on West ing house AP1000 neutronic pa ram e ters. The neutronic cal cu la tions were performed by using the MCNP6 code with the ENDF/B-VII.1 li brary and the new re lease of the ENDF/B-VIII, the AP1000 core with three 235U en rich ment zones (2.35 %, 3.40 %, and 4.45 %). The ob tained re sults showed that the sim u lated model for the AP1000 core satis fies the op ti mi za tion cri te ria as a West ing - house ref er ence. The re sults which in cluded: ef fec tive mul ti pli ca tion fac tor, keff, de layed neu - tron fraction, beff, ex cess re ac tiv ity, rex, shut down mar gin, tem per a ture re ac tiv ity co ef fi cients, whole core de ple tion, neu tron flux, power peak ing factor and core power den sity, were cal cu - lated and com pared with the avail able published re sults. The keff in the cold zero power was found to be 1.20495 and 1.20247 with the ENDF/B-VII.1 and the ENDF/B-VIII li brar ies, re spec tively, which matches the value of 1.205 pre sented in the AP1000 De sign Con trol Docu ment for the UO2 fuel core. On the other hand, keff in the cold zero power was found to be 1.19988 and 1.19860 for MOX core with the ENDF/B-VII.1 and the ENDF/B-VIII li brar - ies, re spec tively, which show good re cep tion and con firm the safety of the de sign and ef fi cient mod el ing of AP1000 re ac tor core.