PURPOSE: To lower the FP gas release rate in nuclear fuel pellets containing penta oxigen niobium.

CONSTITUTION: By sintering a pressed mold body of mixture powder consisting of penta oxigen niobium of 0.5 to 1.0weight% and uranium dioxide, in the week acid atmosphere in the range of oxigen potential between -420 to -500kJ/mol and in the temperature above 1700°C, and making the size of average crystal grain corresponding three dimensional nuclear fuel pellet to over 100μm, nuclear fuel pellets with lowered FP gas release rate are obtained. Therefore, by using these nuclear fuel pellets, the increase of inner pressure in a fuel rod and the reduction of heat conduction in the gap between the pellets and clad are suppressed and fuel soundness can be maintained.