Abstract: The effect of prior particle boundary (PPB) on the crack growth behavior of a powder metallurgy Nickel- based superalloy formed by direct hot isostatic pressing (As-HIP) was investigated. The results show that the precipitated carbon and oxide on the boundary of prior particles in the powder metallurgy superalloy will decrease the fracture toughness (KIC) and initiate cracks from the interface easily. In the course of crack propagation, fractures occur along the inter-particles, which changes the direction of extended edges, aggravates the propagation of cracks along the grain boundaries, and increase the crack growth rate. The fracture toughness of the crack tip extending along the boundary of prior particle is closely linked to the physical properties, number and size of the precipitates on the PPB. The denser the precipitates, the smaller the KIC, and the crack propagation accelerated in these weak areas.