Magnetization reversal in GdCo5single crystals was studied by magnetization measurements at various temperatures. The effective nucleation fieldH*nincreases with decreasing temperature. A linear relationship is observed betweenH*nand the amount of magnetization reversal in a jump at all temperatures studied. The lifetime of the moving walls in the magnetization jump decreases as the temperature falls. The average velocity of the domain walls is estimated to be approximately 18 m/s at 4.2 K. The temperature dependence ofH*nhas been interpreted in terms of a thermally activated process.