A sintered nuclear fuel body includes (U, Pu)O.sub.2 mixed crystals having a mean particle size in a range from 7.5 .mu.m to 50 .mu.m. This sintered nuclear fuel body has a high retention capacity for fission gas in a power reactor. In order to produce the sintered nuclear fuel body by sintering a body in a hydrogen-containing sintering atmosphere, a powered substance selected from the group consisting of aluminum oxide, titanium oxide, niobium oxide, chromium oxide, aluminum stearate, aluminum distearate and aluminum tristearate is added to the starting powder for the body. As an alternative or in addition, the body made from the starting powder is sintered during a holding period of 10 minutes to 8 hours at a sintering temperature of 1400.degree. C. to 1800.degree. C. in a hydrogen-containing sintering atmosphere, initially with an oxygen partial pressure of 10.sup.-10 to 10.sup.-20 bar and then from 10.sup.-8 to 10.sup.-10 and then cooled in a hydrogen-containing atmosphere having an oxygen partial pressure of 10.sup.-10 to 10.sup.-20 bar.