Effects of processing parameters, including the pouring temperature, the die temperature, the ingate velocity, on tensile strength and grain size of die casting Mg alloy AZ91 HP have been investigated. The results show that with the increase of the pouring temperature and ingate filling velocity, the tensile strength is increased firstly and then decreased, while grain size is decreased firstly and then increased. However, with increasing in die temperature, tensile strength of the castings is improved and grain size is decreased. The tensile strength of 224.2 MPa and density of 1.77 g/cm³ of the samples with grain size of 14.4 μm can be observed at the pouring temperature of 680 ℃ and die temperature of 215 ℃ and ingate velocity of 70 m/s. Finally, an empiric formula between the grain size and tensile strength is established through linear regression.