To ascertain the melanosis rule of shrimp during cold storage, the melanosis, serine protease activity and their correlation in Litopenaeus vannamei during 10-day cold storage were investigated. The results showed that the melanosis speed of the shrimp was slow at first and then accelerated, which had no obvious change in appearance until the third day and the most distinct melanosis process was found at the fifth day. This indicated that the fourth day was the key term for melanosis inhibition. Quantitative analysis revealed that there was significant difference in melanosis score among different parts of Litopenaeus vannamei, in which the difference of brightness value L between cephalosome, abdomen and stern became evident at the fourth day and was significant at the fifth day (P<0.05). The correlations of melanosis and serine activity between the three parts were statistically analyzed and appeared that cephalosome was the crucial part for melanosis inhibition due to its most sensitive property to serine protease activity, with slop of k between brightness and serine protease activity of 0.0312, which was larger than that of abdomen (0.0271) and stern (0.0128). This study demonstrated that the prophenoloxidase activating system was discovered in cold storage shrimp, and the level of serine protease activity had similar regulatory effect on the status of prophenoloxidase activating system but its pattern and mechanism remained further study.