Mitsubishi Heavy Industries, Ltd. (MHI) has developed the fuel and core of the MHI's PWR (pressurized water reactor) for burnup extension to increase higher economic efficiency on the basis of its high reliability, and considerable and excellent operating experience. Recently, utilities have applied the 55,000 MWd/t fuels licensed discharge burnup of 55,000MWd/t in assembly average to commercial reactors with excellent results. MHI is promoting the development of further burnup extension to improve higher economic efficiency. MHI is also undertaking the development of fuels and cores with high reliability and flexibility toward the longer cycle and plant uprating operation of commercial reactors.