Ti addition to AZ91 alloy has been investigated with conventional casting. The microstructure and mechanical property were examined. The results show that addition of Ti with an amount of 0.1~0.5%wt resulted in a refinement of the as-cast microstructure. The morphology of β phase is changed from coarse, uneven, semi-continuous skeletal network to small, uniform, short rod-like or granular. When the content of Ti is 0.4wt%, the tensile strength and elongation go up to maximum value of 197 MPa and 6.9% respectively. Small addition of 0.2%wt Ti to AZ91 alloy predominantly forms TiAl3 phase, lying in β-phases at the grain boundaries. The mechanism of mechanical properties improvement caused by Ti addition is discussed.