The objective of this study was to investigate the dependence of the room temperature tensile properties on the volume fraction of discontinuous precipitates (DPs) in a cast AZ91 magnesium alloy. In order to obtain various volume fractions of DPs, the solution-treated alloy was aged at 428 K for up to 48 h. The volume fraction of DPs increased from 0% to 72% with an increase in the aging time up to 24 h; for aging times longer than 24 h, discontinuous precipitation was substantially inhibited owing to the occurrence of significant continuous precipitation within the α-(Mg) grains. YS and UTS of the alloy increased with the volume fraction of DPs, whereas the elongation showed a reverse trend. A relatively rapid change in the tensile properties with increasing volume fraction of DPs up to ~40% was noted, which would be due to the reduction of the effective α grain size in response to the formation of DPs along the grain boundaries.