The effect of strontium (Sr) on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy and its sensitivity to cooling rate are investigated. Three phases - blocky-shaped Mg Al, acicular MgA lMnSr, and insular Mg(Al, Zn)Sr - are identified in the Sr-containing AZ31 alloys. With increasing cooling rate, the blocky-shaped MgA l phase increases, the acicular MgA lMnSr phase diminishes, and the insular Mg (Al,Zn)Sr phase is refined and granulated. The study suggests that the grain size decreases with increasing cooling rate for a given composition. However, the grain size decreases first, then increases, and finally decreases again with increasing Sr for a given cooling rate. The yield strength (σ) of AZ31 magnesium alloy can be improved by grain refinement and expressed as σ = 35.88 + 279.13d according to the Hall-Petch relationship. The elongation increases when Sr is added up to 0.01 pct and then decreases with increasing Sr addition. Grain refinement changes the fracture behavior from quasicleavage failure for the original AZ31 alloy to mixed features of quasicleavage and microvoid coalescence fracture.