Abstract: In order to determine the endogenous protease that results in muscle softening and its distribution in Litopenaeus vannamei,different endogenous proteases activities from the shrimp were examined,as well as the changes of main proteases during ice storage and the proteolytic degradation of myofibrillar protein by endogenous proteases were monitored in this study.Results showed that proteolytic activity in the shrimp head was up to 90.17 U / mg,300 times higher than that in muscle. In shrimp head,the trypsin was the most important proteinase,with 13.59 U / mg activities. In addition,the increase of total proteolytic and trypsin activities in shrimp muscle were detected after 4 days of storage,and the total proteolytic activity in first segments increased to1.66 U / mg after 8 days of storage,while trypsin activities were 0.97 U / mg and 1.84 U / mg when BAp NA and TAME were used as substrate,respectively.In the results of SDS- PAGE,the degradation of MHC and Actin in myofibrillar protein by endogenous proteases was enhanced with increasing storage temperature and time. Among them,the trypsin had the most remarkable effect on the degradation of myofibrillar protein.Therefore,these results indicated that the release of trypsin from Litopenaeus vannamei shrimp head was most likely responsible for the undesirable softening of the shrimp muscle during post- mortem storage.