Aluminum alloy castings have been widely applied in automotive and aerospace fields. One of themost common defects of cast aluminum alloys is pore, which often results in poor mechanical properties suchas limited strength and ductility, short fatigue life and increasing variability in properties. In this investigation,micropores in practical ADC12 aluminum high pressure die castings were detected with 3D high resolution X-raycomputed tomography technology. The 3D morphologies, volumes, surface areas and sphericity coefficients ofthe pores were presented and analyzed. The pore's general characterization was summarized. Based on theirdifferent morphologies and characteristics, there were three types of pores, namely, gas, gas-shrinkage andshrinkage pores, detected in the die casting alloys. The volumes, sphericity coefficients and pressures of the threetypes of pores were compared and discussed. Then their formation mechanism was proposed. It was concludedthat the gas pore was with low volume, high pressure and near round shape. The appearance of shrinkage-gaspore was sphere with convexes or long tails. Compared with the gas pore, the shrinkage-gas pore volume washigher while its pressure was lower by one order of magnitude. For the shrinkage pore formed during the highsolid fraction period, it was characterized with low volume and tortuous and complex shape in space. The sphericity and pressure of the shrinkage pore were the lowest among the three types of pores and its pressure wasapproximate three orders lower than that of the gas pore. Furthermore, the volume distribution of pores wasinvestigated by statistical analysis, which showed that the three-parameter lognormal can fit the volume distribution better than lognormal.