X-ray tomography was used to characterize the porosity in high pressure die casting of AM60B alloy. In situ tensile deformation was performed to observe the change of porosities and their influences on crack initiation,propagation and subsequent fracture of specimen. Results showed that four types of porosities,including gas-shrinkage pore,gas pore,net-shrinkage and island-shrinkage,could be identified according to the formation mechanism and morphology characterization. During tensile deformation,it was shown that the gas-shrinkage pore and net-shrinkage,rather than gas pore or island-shrinkage,were the main sources for crack initiation. In addition,the crack propagated by interconnecting the porosities at the cross section with minimum efficient force bearing area. At these locations where externally solidified crystals(ESCs) were present,the crack would propagate along the ESC boundaries in an intergranular mode,while at these locations without ESCs,the crack would propagate roughly along the direction perpendicular to the tensile stress in a combination of trans-granular and inter-granular modes. X-ray tomography was used to characterize the porosity in high pressure die casting of AM60B alloy. In situ tensile deformation was performed to observe the change of porosities and their influences on crack initiation, propagation and subsequent fracture of specimen. Results showed that four types of porosities, including gas-shrinkage pore, gas pore, net-shrinkage and island-shrinkage, could be identified according to the formation mechanism and morphology characterization. During tensile deformation, it was shown that the gas-shrinkage pore and net-shrinkage, rather than gas pore or island-shrinkage, were the main sources for crack initiation. In addition, the crack propagated by interconnecting the porosities at the cross section with minimum efficient force bearing area. , the crack would propagate along the ESC boundaries in an intergranular mode, while at these locations without ESCs, the crack would propagate roughly along the direction perpendicular to the tensile stress in a combination of trans-granular and inter-granular modes.