Effects of Si content and modification on microstructure and properties of in-situ Mg2Si/Mg composites prepared with Mg-Si binary alloy have been investigated. The results reveal that with increasing the Si content, the fluidity of Mg-Si alloy is decreased, resulting in the increase of in-situ Mg2Si particles in the composites and the generation of more developed dendrite, such as Chinese script. Although the modification of optimized Mg-8Si alloy with MM（mistchmetal） and Sb respectively all can improve the distribution and shape of Mg2Si particle, the modification effect with Sb is obviously superior to that with MM. With addition of 1.2% Sb, the Mg2Si particle can be transformed from the Chinese script to uniformly dispersed particles in the composites, greatly improving the wear resistance of the composites.