In traditional pressurized water reactors fuelled with uranium oxide (UOX), extended burnup can be achieved through the use of a high uranium enrichment and high percentage of gadolinium as burnable poison. Such a high enrichment of fuel and gadolinium would lead to increasing fuel cycle costs. Consequently, searching for another burnable poison that can achieve a long core cycle and at the same time may be mixed with nuclear fuel with lower concentration was one of the main interests of nuclear power plants in recent years. Erbium is one of the burnable poisons that can satisfy these goals. The potential interest of this poison (Er-167) is that it presents a relatively low absorption cross section in the thermal range and a non-negligible resonance integral. This will lead to very long cycles owing to its slow consumption in the reactor core. The main objective of this work is to investigate the effect of mixing erbium oxide with uranium oxide on some neutronic safety parameters of IRIS (International Reactor, Innovative and Secure) assembly. Among these parameters are the effective multi-plication factor, the thermal flux, the total flux, the residual reactivity penalty, the radial power and the axial power distributions, the fuel temperature coefficient, the fuel temperature coefficient and peaking factor. Three assembly models are simulated using MCNPX 2.7 code; the first model uses uranium oxide fuel with no Erbium (4.95 % U-235), while the other two models use Erbium with different concentrations (1% Er2O3 and 2.2 % Er2O3). The obtained results for k-effective showed that the suggested loadings of the Erbium bearing pins can sustain very long burnup cycles up to 60 GW d/t. It is also noted that the assemblies containing erbium have higher amounts of U-235 at 60 GW d/t (the end of life). Furthermore, erbium models present a higher spectral index than that of the free Erbium model. Additionally, it is observed that there is an increase in the pin power peaking factors for erbium assemblies. Finally, FTCs and MTCs are more negative for assemblies bearing erbium especially for the model containing 2.2 % Er. This guarantees the safe operation of the reactor.