The Mg-5%Si is a hypereutectic alloy that the formation of Mg 2 Si phase during solidification fell it in to the category of non-reactive in situ metal matrix composites. However, good properties of this composite is dramatically influenced by the tendency of Mg 2 Si phase to form coarse Chinese script or dendrite shape under normal cast condition, which results in the low ductility and strength of the composite and restricts its application. In view of the above, an attempt has been made to refine microstructure of the in situ composite of Mg-5Si by chemical, thermal and chemical-thermal methods. To fabricate Mg/Mg 2 Si composite, industrially pure Mg ingot and Si were used as starting materials. The melting process was carried out in a low carbon steel crucible by using an electric resistance furnace under protective argon atmosphere which also stirs the melt. To fabricate modified samples, different amounts of Bismuth and Zinc (0.5 to 1.5 % mass fraction) were added in to the Mg–Si melt as chemical modifiers. The effect of chemical modifiers content on microstructure of Mg 2 Si/Mg composite was investigated by optical and scanning electron microscopy followed by EDX analysis and X-Ray diffraction. Quantitative image analyze resulted that by addition of 1% Bi or Zn the following changes in microstructure would happen: 1- Decreasing the average size of Mg 2 Si from more than 100?m to about 20?m or less. 2- Changing the morphology of Mg 2 Si from dendritic shape to polyhedral shape 3- More even distribution of Mg 2 Si in the matrix and reducing of clustered polyhedral Mg 2 Si. However, when the modifier content exceeded 1%, it is found that many primary Mg 2 Si particles became coarse again and over modification had occurred which is a complicated and unknown phenomena in Mg-Si and Al- Mg- Si systems.The effect of cooling rate on modifying microstructure was investigated by the use of a directional solidification system. Structural characterizations of directed solidified Mg/Mg 2 Si, Mg/Mg 2 Si+1%Bi and Mg/Mg 2 Si+1%Zn samples were carried out on four thermal sections of each fabricated. The Results showed that an increase in cooling rate cause an almost same modification effect on samples with chemical modifiers. Although the structure of Bismuth and Zinc modified samples effected in a good way by higher cooling rates, there were no evident of modified Mg 2 Si particles in samples with almost low cooling rates. This may happen by segregation of dense modifiers due to longer solidification time or weakening effect of low heat transfer on chemical modification mechanism. It also worth to mention that samples which were affected by both zinc chemical modification and thermal modification exhibited finer and fiber shape eutectic instead of former flake like. In the next step, hardness, wear resistance, compression and tensile strength of selected modified and unmodified samples were investigated. The results showed that modified structures have better mechanical and wear properties than unmodified samples. However the amount of this enhancement changes from minor to major amounts due to the nature of each test .Best results were belonged to tensile examination results were modified structure has shown 11% enhancement i ultimate tensile strength. Keywords In-situ Mg/Mg 2 Si composite, chemical modification, Thermal modification, Directional solidification, Mechanical properties, Wear resistance.