Shrimp head of Litopenaeus vannamei were autolyzed under the optimized condition,the hydrolysate products were determined by HPLC for the characterization of molecular weight distribution and the composition of free amino acids.The results showed that during the autolysis(0-5 h),the release quantity of free amino acids increased gradually while fatty amino acid increased more sharply.Aspartic acid and glutamine contents increased more than four times,while the contents of tryptophan,heterocyclic amino acids and sulfamino acid kept unchanged.The distribution profiles of molecular weight(MW) exhibited that a majority was the substance with MW over 10000u before autolysis;the substance with MW under 3000u remarkably increased during 1～3h and the increasing trend slowed down after 3h;and 85% of the substances with MW over 10000u were degraded into small peptides and free amino acids after 5h.