The rheological behavior of semi-solid AZ91 D magnesium alloy was investigated in isothermal steady state condition. The effects of stirring temperature and shearing rate on apparent viscosity of semi-solid alloy slurry at steady state were discussed. The results show that the apparent viscosity of semisolid AZ91 D alloy increases with increasing solid fraction. It increases slightly before the solid fraction reaches a certain value, about 0.4, and then goes up rapidly after the solid fraction reaches the critical value. However, the apparent viscosity decreases with increasing shearing rate, and the reduction amplitude is higher when the solid fraction is higher. According to the experimental data, an empirical equation that shows the effect of solid fraction and shearing rate on the apparent viscosity of semi-solid AZ91 D alloy can be built as. The rheological behavior of semi-solid AZ91 D magnesium alloy was investigated in isothermal steady state condition. The effects of stirring temperature and shearing rate on apparent viscosity of semi-solid alloy slurry at steady state were discussed. The results show that the apparent viscosity of It increases slightly before the solid fraction reaches a certain value, about 0.4, and then goes up rapidly after the solid fraction reaches the critical value. However, the apparent viscosity decreases with increasing shearing rate, and the reduction amplitude is higher when the solid fraction is higher. According to the experimental data, an empirical equation that shows the effect of solid fraction and shearing rate on the apparent viscosity of semi-solid AZ91 D alloy can be built as.