Abstract: In the first cycle loading of the AP1000 reactor, assemblies with five kinds of enrichments and burnable poisons such as IFBA, WABA are employed. For some assemblies, there are regeneration blankets at both ends. All these make the core arrangement extremely complex. The lattice physics calculation was performed using the KENO 3-D Monte Carlo criticality computer code and NEWT multigroup discrete ordinates radiation transport code of SCALE code system. The k_inf and atomic number density of typical isotopes as a function of burnup for different enrichment fuel cells and fuel assemblies were studied on the condition of the deep burnup, and the physical characteristic of AP1000 assembly was concluded.