The viscosity and microstructure evolution of A356 AI-Si alloy in the semi-solid state were investigated. During continuous cooling, the viscosity increases slowly at the onset of the semi-solid region, and starts to increase abruptly after reaching a critical temperature. The higher the shear rate, the lower is the critical temperature. During isothermal stirring, the viscosity decreases and the sphericity increases as the shear rate increases. The grain size decreases and the sphericity increases as the shear rate increases for short stirring times. The viscosity increases as the temperature decreases during isothermal stirring. The spheroidizing effect is not significant for short stirring time. During isothermal stirring, the viscosity increases as the rest time increases. Both the grain size and sphericity increase as the rest time, following stirring, increases. The coarsening exponent is 3.2, suggesting that the coarsening is controlled by the volume diffusion of the grains. During isothermal stirring, the viscosity decreases as the stirring time increases. The sphericity increases and the grain size decreases as the stirring time increases. We propose that there is a critical stirring time, above which the grain size increases with stirring time, but below which the grain size decreases with stirring time.