The present invention relates to a process for sintering a compacted powder of at least one oxide of a metal selected from an actinide and a lanthanide, this process comprising the following successive steps, carried out in a furnace and under an atmosphere comprising an inert gas, dihydrogen and water: (a) a temperature increase from an initial temperature Ti up to a hold temperature TP, (b) maintaining the temperature at the hold temperature TP, and (c) a temperature decrease from the hold temperature TP down to a final temperature TF, in which the P(H2)/P(H2O) ratio is such that: 500 < P(H2)/P(H2O) <=50 000, during step (a), from Ti until a first intermediate temperature Ti1 between 1000 DEG C and TP is reached, and P(H2)/P(H2O) <= 500, at least during step (c), from a second intermediate temperature Ti2 between TP and 1000 DEG C, until TF is reached. The invention also relates to a process for manufacturing a pellet of at least one oxide of a metal selected from an actinide and a lanthanide, especially a nuclear fuel pellet.