The thermal oxidative degradation of polypropylene-impact copolymers (ImPP), which was separated into three fractions, I (105°C-110°C), II (110°C-115°C) and III (115°C-145°C), using temperature rising elution fractionation (TREF) was investigated. The primary structure of each fraction, e.g. copolymer compositions and microtacticities of PP sequences, was determined by FT-1R and 13C-NMR. It was found that the fraction of higher temperature range (115°C-145°C) was mainly composed of homopolypropylene, containing much meso pentad (mmmm) sequence, and was the matrix of ImPP. The fraction showed less stability for thermal oxidative degradation, suggesting that the degradation behavior of ImPP was determined principally by the content of this high mmmm fraction.