The tribological properties of the magnesium composite alloys reinforced with solid-state synthesized Mg2Si or Mg2Si/MgO dispersoids are evaluated under wet and dry conditions by using pin on disc type wear test equipment. In the wet type wear test, every composites show a dependence of the friction coefficient on the applied load, which corresponds to Sribeck diagram based on elasto-hydrodynamic lubrication (EHL). The increase of Mg2Si content causes the increase of both the friction coefficient and the total wear by plowing, because Mg2Si dispersoids protruded from the surface are much harder than S35C mild steel counter material. When including MgO dispersoids formed in deoxidizing SiO2 particles by magnesium, the friction coefficient reduces to 0.01-0.02 in oil lubricant. This is due to the "mild offensive effect" by MgO dispersoids which ease the attacking or plowing phenomenon on the counter material, because they are not so hard. In the dry type wear test when using the ADC12 aluminum alloy disks as the counter material, Mg2Si and MgO dipsersoids also show the same effect to reduce the abrasive wear phenomenon. It is also clarified that the graphite particles embedded in the magnesium matrix prevent the fluctuation of the friction coefficient by the formation of the self-lubrication films between the pin and disk specimens.