Carbide reactions in 1.5Cr—1.0Mo—0.3V steel are investigated at various thermal treatments at 550–650°C. The cooling rate control was carried out by cooling the environment. Metal tempering was conducted in the range 650–720°C for up to 100 h. It is shown that the rate of preliminary cooling from the austenitic region has the principal effect on the carbide reaction nature. At rapid cooling (martensite structure) the major carbide reaction during tempering and ageing is M3C→M7C3whereas for cooling rate decreasing the reaction M3C→M23C6prevails. Tempering and ageing change the quantitative characteristics of the carbide state without having an effect on the major mechanism of transformation. The carbide reaction character is compared with metal properties under the same time—temperature conditions and the correlation between the structural state and metal service properties is examined.