[en] The basic study of the behavior of high burnup MOX fuel was carried out with simulated MOX fuels which were made by (U_0.8, Ce_0.2)O₂, and following conclusions clarified. It was clear that thermal conductivity of simulated burnup MOX fuel, (U_0.8-y Ce_0.2 My)O_2±x [M : Nd or Zr] (0≤y≤0.13), depended on temperature and concentration of added elements, and thermal conductivity of simulated burnup MOX fuel could be expressed by the equation with that of (U_0.8, Ce_0.2)O₂ and the concentration of added elements, approximately. It was clear that elastic constant, Vickers hardness and yield stress of simulated burnup MOX fuel decreased with the increase in concentration of added elements for the measurement of sound velocity and Vickers hardness of samples. The prediction of the physical property of fuel carried out with the molecular dynamics method, and the chemical form of FP in high burnup fuel were estimated with ChemSage which was the calculation program of thermodynamics. The results of those two methods were reasonable about simple systems. (author)