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The effect of muscle fiber type on protein degradation of Xinjiang brown cattle beef during postmortem aging was observed. Different cuts of longissimusdorsi (LD), psoas major (PM), biceps femoris (BF) with different muscle fiber type composition were chosen to measure protein solubility, myofibril fragmentation index, muscle fiber ultrastructure and SDS-PAGE respectively at 1h, 6h, 12h, 1d, 3d, 7d and 14d after slaughter. The samples were kept at 4℃ during postmortem aging. The results indicated that protein degradation patterns of Xinjiang brown cattle beef cuts with different muscle fiber type composition were definitely different. The initial MFI of psoas major with higher muscle fiber type Ⅰcontent, was higher than the initial MFI of longissimusdorsi and biceps femoris with higher muscle fiber typeⅡcontent, but the muscle fiber degradation rate of psoas major was lower than the rate of longissimusdorsi and biceps femoris. The significant positive correlation existed between the content of muscle fiber type ⅡA and the initial protein solubility, and significant negative correlation existed between muscle fiber type ⅡA content and protein solubility change rate. The results of muscle fiber ultrastruture and SDS-PAGE research were consistent with those provided by MFI and protein solubility. Ultrastructural investigation indicated that proteins of longissimusdorsi and biceps femoris degraded faster than proteins of psoas major. The degradation patterns of troponin-T and desmin were also different between the three beef cuts. The troponin-T and desmin degradation rate of the beef cuts with high muscle fiber type Ⅱ content was faster than that of the beef cuts with high muscle fiber typeⅠcontent. Protein degradation patterns of Xinjiang brown cattle beef with different muscle fiber type composition were definitely different during postmortem aging. The results would provide a theoretical basis for the regulation of postmortem aging process according to the different muscle fiber types and thus the development of highgrade Xinjiang brown cattle beef.