The characters of semi-solid AZ91 magnesium alloy has been investigated under the different temperature, different shear rates and different cooling condition. Two cooling conditions are quenching in water and gravity casting in metal molds. Shear rates are changed from 110s(-1) to 330s(-1) in the experiments. Research results show that the shear rates give a big influence on the grain density of the alloy. The optimum value for shear rate to make the finest grain is 220 s(-1) in these,experiments. The stirring temperature influences the grain density. The suitable stirring temperature is between 550degreesC to 570degreesC for AZ91 Mg alloy. Stirring times as little as 3 minutes can create non-dendritic semi-solid slurries, longer stirring time is not need for the enhancement of grain density. The grain density of the sample cast in metal mold is less than that of sample quenched in water, because the grains have trend of growing and merging with each other in the cooling process.