The effect of microporosity on the variability in the tensile properties of high-pressure die-cast AM60 and AZ91 alloys was investigated, together with a theoretical prediction based on a constitutive model. The strain rate sensitivity of both alloys was measured through an incremental strain rate change test at room temperature, and the microporosity was measured through quantitative fractography analyses on the fractured surface. The variability in the tensile strength and elongation of both alloys can be empirically described as a power law relationship in terms of the microporosity variation. The defect susceptibility of the UTS and elongation to the microporosity variation in the AZ91 alloy is slightly higher than that in the AM60 alloy. The constitutive prediction on the tensile properties of the AM60 and AZ91 alloys is in good agreement with the experimental results, and it suggests that the defect susceptibility of the tensile properties to the microporosity variation is significantly decreased with the increase of strain hardening exponent.