The improvement of thermal conductivity of tool steel is a very important problem to achieve life prolongation of a metal die used in press forming and die-casting and obtain large size or complex shape products with good mechanical properties. To improve the thermal conductivity without the degradation of mechanical properties, some kind of dispersants were added into tool steel 40CrMoV5 in ISO standard. 0.5–1.0 vol. % carbon nanofiber, 4–8 vol. % TiB2 particles with 2.62 micrometer in average diameter and 8 vol. % Cu particles was added in tool steel. Dense composites were fabricated by spark plasma sintering by controlling sintering temperature and time. The thermal conductivity was improved by adding all dispersant. Strength of carbon nanofiber / steel composites increased by adding 0.5 vol. % fiber, but decreased by adding 1.0 vol. % because of aggregation of carbon fiber. Some chemical reaction occurred in TiB2 particles/ steel composites, and the elongation improved because of the boron element in the interface. Cu /steel composites keep good strength compared with monolithic tool steel and the thermal conductivity increased dramatically as increasing Cu contents.