The purpose of this research is to study the influence of trace Titanium (Ti) on the microstructures and properties of as-cast Mg-9Al alloy. We found that Ti could increase the corrosion resistance of the alloy in NaCl solution. The addition of trace Ti in Mg-9Al cast alloy improved the properties greatly. The elongation (delta(5)) of the alloys was also improved greatly with Minor addition of Ti as low as 0.015wt%. The improvements on the mechanical properties and corrosion resistence were mainly due to the inhibition of Ti to the heterogeneous nucleation during solidification and the modification of beta phase. As the Ti additional amount increases, the grain of as-cast Mg-9Al alloy coarsens rapidly. Meanwhile, the beta phase was refined and changed from semi-netted shape to short rods or even spheres.