A core inlet enthalpy controlled core and its control system are proposed. This core can flatten the axial power distribution even at partial power and improves the core performance for startup operation 1 of boiling water reactor plants. The feedwater enthalpy is controlled to make the core inlet subcooling constant throughout the startup operation. With this control, the increase of axial power peaking in the lower part of the core at partial power can be suppressed. The core inlet enthalpy controlled core is expected to reduce the time required for the startup operation by about 38% compared with the core evaluated without inlet enthalpy control. The control system of the axial power distribution for the core inlet enthalpy controlled core can also be applied to the load follow operation.