Abstract: Different silicon coupling agents ( SCA) cause a large difference in mechanical properties of the glassfiber reinforced MC nylon ( GFRMCN) . The KH-570 SCA makes them decrease while the KH-550 makes themimprove. There is a rational formula between the mass f ractions of KH-550 and glass fiber. When the mass f ractionof KH-550 is 0. 2 % , the bend st rength , bend modulus , tensile st rength , tensile st ress and impact st rength ofGFRMCN are increased by 35 % , 72 % , 46 % , 88 % and 41 % , compared with those of MC nylon. Glass fibers af tersurface t reatment with KH-550 SCA increase the properties of GFRMCN because of a good interface between GFand nylon. However , most glass fibers are drawn out f rom nylon basement if they are not t reated with SCA. Theinterfaces of GF/ nylon become fatal weakness and reduce the properties of GFRMCN.