There is disclosed a process for fabricating UO.sub.2 pellets from highly active uranium dioxide powders, comprising the steps of (1) uniformly adding at least one pore-former agent in the range of 0.3 to 1.4% by weight to uranium dioxide powders as a starting material, the pore-former agent decomposing and sublimating below 600.degree. C. and having an average particle size in the range of 5 to 500 .mu.m, (2) compacting the uranium dioxide powders including the pore-former agent to form green pellets, and (3) sintering the green pellets to form sintered UO.sub.2 pellets having large grain size crystals. According to the present invention, UO.sub.2 pellets having the sintered density in the range of 94 to 97% TD are fabricated from highly active UO.sub.2 powder when the pellets have large crystalline grain sizes exceeding 20 .mu.m. Accordingly, the fission gasses generated during irradiation is restrained to the inside pellet thereof to enhance the safety of the irradiation.