Fracture toughness tests of P/M high speed steels with different carbide sizes were carried out by using three point bend specimens with fatigue precrack. Tensile and bending strength tests were also carried out. The heat treatment to control the size of carbide particle also resulted in the change of volume fraction of carbide particles: the volume fraction of carbide particles decreased with an increase in the size of carbide particle. These two competitive factors, the size and volume fraction of carbide particles, simultaneously influenced the fracture toughness of the product in such a way that smaller size and higher volume fraction result in lower fracture toughness. Tensile and bending strengths increased with reducing the size of carbide particle. In the tensile and bending tests, fracture occurred from the site where some carbide particles or inclusions were segregated. Since the size of the initiation site was in proportion to the size of carbide particle, it is apparent that the both strengths increased with reducing the carbide particle size.