If a loss-of-coolant accident occurs in a fusion reactor, the temperature in the vacuum vessel will rise. If the decay heat is not removed, then the plasma vacuum boundary may melt. In this paper, the effects of the decay heat in a LOCA are analysed numerically based on the Fusion Experimental Reactor (FER). ... the fusion power: ∼ 460 MW. In the case of a loss-of-coolant accident with the plasma shut down, it is assumed that the decay heat is removed by the radiation of the divertor only. If the radiant effect is a quarter of the black radiation, the divertor plate will not melt, because the temperature rise is less than 150 K. Secondly, it is assumed that the decay heat is removed by the radiation between the outer shield and the heat shield. When the initial shield temperature is low and the amount of the shield is large, the temperature rise is negligible, because the heat capacity is large enough to absorb the heat. The effect of the natural convection outside of the cryostat is negligible to remove the heat.