In this paper, a commercial impact-resistant polypropylene alloy sample was fractionated with preparative temperature rising elution fractionation (t.r.e.f.) technique and a series of ethylene-propylene block copolymer fractions with different ethylene contents were obtained. Combined with fractionation results, the block structure of them was identified with C-13 nuclear magnetic resonance (n.m.r.) spectroscopy and differential scanning calorimetry (d.s.c.). There are some transition segments between polyethylene and polypropylene segment in the block copolymer. It was found that the number and kind of transition segments decreased with elution temperature, the length and melting temperature of polyethylene segment in block copolymer fractions showed a similar tendency.