Changes in microstructure and viscosity with both stirring conditions and a fraction of solid have been investigated in order to obtain fundamental aspects of various poroperties of a semi-solid stir-cast AZ91D magnesium alloy. Mechanical stirring was conducted during cooling of a liquid phase into mushy zone at a given temperature. Tensile test was performed for as-cast and T6-treated specimens. The primary α phase in specimens changed from the rosettelike α phase, whose dendrite arms were bent by stirring, to spheroid during the isothermal stirring. The spheroidization of the primary α phase and the decrease in their size were accelerated with the increase in stirring speed, stirrer width and the fraction of liquid. The stirring torque during the isothermal holding decreased in accordance with the spheroidization of the α phase. 0.2% proof stress of as-cast specimens was higher than the minimum value specified in JIS because of a fine eutectic phase crystallized by rapid cooling. Tensile properties of both as-cast and T6-treated samples had a tendency to decrease with increasing the fraction of solid. Tensile stress increased with the T6 heat treatment and the increment became higher with the fraction of solid being lower during isothermal stirring.