Because　of　high　melting　point　(2300　K),　rather　low　density　(6.28g/cm(3))　and　excellent　elevated　oxidation　resistance　(above　1600　degrees　C),　MoSi2　have　been　treated　to　be　one　of　the　most　attractive　high　temperature　materials.　However,　thus　a　MoSi2　materials　have　their　intrinsic　limitations,　one　of　the　worst　things　is　it　suffered　from　an　accelerated　oxidation　at　low　temperatures　(between　350　and　550　degrees　C)　which　was　first　discovered　by　Fitzer　in　1955　and　was　named　PEST　oxidation.　The　phenomena　of　PEST　extremely　hindered　the　application　of　MoSi2　in　advanced　engineering　and　aerospace,　also　limited　its　longevity　and　safety.　In　this　paper,　an　in-situ　MoSi2-SiC-MoB　three-phase　composite　was　prepared　by　reactive　infiltration　process　which　is　known　as　low-cost　and　net　shape　method,　and　after　the　oxidation　process　in　500　degrees　C　for　3　h,　it　was　found　that　there　is　no　PEST　occurred　anymore.　And　the　residual　bending　strength　of　this　composite　will　still　over　68%.　The　reasons　for　overcoming　PEST　would　be　the　in-situ　formed　MoB　phase　will　be　stiffly　covered　over　the　surface　of　MoSi2　grains,　and　the　volume　expansion　of　SiC　phase　and　volume　contraction　of　MoB　phase　will　be　matched　in　the　composite　system　9.7%wt　MoB/8.6%wt　SiC/MoSi2.