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Lysosomal cathepsins are enclosed in the lysosomes, which could be released into the cytosol as a consequence of lysosomal disruption occurring after cell death. It was approved that cathepsins played an important role in the degradation of myofibrillar protein in beef during postmortem aging. Despite its significance, there was little information about the correlation between cathepsins and muscle fiber types in beef during postmortem aging. The objectives were to compare the results of ATPase staining and RT-PCR in typing muscle fibers, find out the relationship between muscle fiber type and the activity of cathepsins, and reveal the possibility that muscle fiber types affect meat quality through cathepsin activity. Eight different parts of the muscles (Semitendinosus, Latissimus dorsi,Longissimus thoracis,Infraspinatus,Biceps femoris,Gastrocnemius, Rhomboideu and Psoas major) in Xinjiang brown cattle were removed from the left side of each carcass. The muscle fiber type composition, fiber diameter and fiber crosssectional area were detected by ATPase staining, and MyHC gene expression of each muscle was determined by RT-PCR. The changes of cathepsin B, L, H activity were investigated at 0d, 3d, 7d, 9d, 11d and 14d during postmortem aging, respectively. The results showed that the composition of muscle fiber types in different parts of Xinjiang brown cattle was quite different. There was a significant correlation (p＜0.01) between the two methods when they were used to quantify the muscle fiber type Ⅰ and MyHC Ⅰ gene expression. However, muscle fiber type Ⅱ and the expression of MyHC Ⅱ gene could not match very well. The proportion of muscle fiber ⅡB was negatively correlated (p＜0.05) with the expression of MyHC Ⅱa and positively correlated (p＜0.05) with the expression of MyHC Ⅱx. MyHC Ⅱb was not found in eight muscles of Xinjiang brown cattle. It could be inferred that there was a correlation between the expression pattern of MyHC gene and the location of muscle. The activity of cathepsin was increased gradually during senescence after death and reached its highest value after 9 days. The content of cathepsin H in bovine muscle was extremely rare. Cathepsin L’s activity was about 2.5~4 times of cathepsin B’s activity, and more than ten times of cathepsin H’s activity. There was a significant positive correlation (p＜0.05) between the proportion of muscle fiber Ⅰ and the activities of cathepsin B and L after 9 days and 11 days. Further investigation was necessary to identify the function of cathepsins in the relationship between muscle fiber type and tenderness of beef.