Abstract: A method of compacting radioactive metal wastes comprising enclosing the radioactive metal waste in vacuo in a capsule, placing the capsule into a pressure container, and subjecting the capsule to an increased pressure at a high temperature to compact the waste. The compacting treatment is conducted in a state in which the atmosphere within the pressure container contains water molecules in an amount, in terms of the total weight W (g) thereof, the amount being preferably in the range of:1.3.times.10.sup.-6 .times.V.ltoreq.Wwherein V (cm.sup.3) is the volume of the compacting space in the pressure container. Alternatively, an oxide coating is formed on the outer surface of the capsule before the capsule is placed into the pressure container, or the formation of the oxide coating is followed by the compacting treatment conducted in the above-mentioned state.