Abstract: The fracture toughness K_IC and dynamic fracture toughness K_Id and the fracture micromorphology of sintered and vacuum heat-treated heavy alloy (95W-3.5 Ni-1.5 Fe) specimens have been studied. The results show that the K_IC and K_Id of vacuum heat-treated alloys are increased with the increase of tensile strength and elongation at ambient temperature. Because heavy alloys have the high sensitivity to loading velocity, the K_IC of as-sintered and as heat-treated specimens is higher than K_Id. Therefore, it is reliable to evaluate the size of a by the value of K_Id when the alloys are used under dynamic conditions. Meanwhile,the cohesion between the interface of tungsten particle phase and bonding phase is on the increase by vacuum dehydrogenization with the result that the cleavage facets of fracture surfaces of vacuum heat-traeted K_IC and K_Id specimens increase. The increase of loading velocity causes that the cleavage facets of fracture surface of K_Id specimens are more than those of K_IC.