Sulphidation method for a UO.sub.2 powder, in which said powder is sulfurated by bringing it into contact with a gaseous sulphidation agent.Method for manufacturing nuclear fuel pellets based on uranium oxide, or mixed oxide of uranium and plutonium, from a load of totally or partially sulfurated UO.sub.2 powder or UO.sub.2 powder and PuO.sub.2 powder, by lubrication, pelletizing and sintering, in which: the load of powder subjected to the lubrication, pelletizing and sintering is prepared by the following successive steps: sulphidation of a UO.sub.2 powder by the above sulphidation method; optionally mixing, said sulfurated powder in a matrix comprising a UO.sub.2 powder, or of a UO.sub.2 powder and a PuO.sub.2 powder; and, subjecting said load, formed from said sulfurated powder or said mixture, to lubrication, pelletizing and sintering operations.