rystalline phase compatibilities between LLDPE,HDPE and LDPE was studied through DSC,WAXD,IR and tensile test.It is found that LLDPE which has similar chain structure as HDPE,is compatible with HDPE in crystalline state.While LDPE,which has more different chain structure,has poor compatibility with HDPE. HDPE and LDPE chains are inclined to crystallize separately in the existence of partial chain interpenetration Not only LDPE chains which have larger number of branches penetrating into HDPE crystal,but also HDPE chains,which have smaller number of branches,penetrating into LDPE crystal,can destroy the regularity of the crystal.The formation of cocrystal make the melting points,crysrallinities and crystal dimensions of the blends to be smaller than the linear sum of the two components.Whereas the mechanical properties,espectially in elongational characteristics,improve dramatically.