Difference in thermal behaviour of presumed polypropylene-b-polyethylene block copoly-mers (PP—PE) and corresponding PP+PE blends was studied．Different views jn the litera-ture were unified in our observation that faster cooling rate yielded only one exothermal peak for the blends，while slower cooling rates revealed both PP and PE exothermal peaks．Further details on when a single or double exothermal peaks would appear are discussed．Melting and crystallization temperatures for both PP and PE in blends were found to be a few degrees higher than for PP and PE in block copolymers．Thus，thermal analysis can be used to identify PP-PE block copolymers．These phenomenon and the lower △H-values of PP and PE in block copolymers than the △Hr-values of pure homo-PP and -PE (for PE even more so) are explai-ned in terms of restricted block movement due to covalent bond between blocks and of crystalli-zation processes in block copolymers．The presence of block structure in the PP-PE samples studied is inferred．