Objective To explore the effect of morphine hydrochloride on growth accumulated degree hour(ADH)and cephalopharyngeal skeleton of larvae of Parasarcophaga crassipalpis under natural condition, and to prove the feasibility of predicting the postmortem interval(PMI)using ADH and the shape indexes of cephalopharyngeal skeleton. Methods The larvae of Parasarcophaga crassipalpis were fed under natural condition with rabbit meat injected with morphine hydrochloride before death. The larvae were collected at 12 h intervals until the end of the stage(the larvae did not eat any more). The larvae were fixed in boiling water and were stored as specimen. The average daily temperature during the experiment was calculated for ADH. Then the cephalopharyngeal skeletons were isolated, observed, and then photographed under light mircoscope. Finally the morphology of the cephalopharyngeal skeleton was analyzed by digital image processing system. Results (1)Under natural conditions, the ADH was changed by morphine hydrochloride to a certain degree.(2)The area of the cephalopharyngeal skeleton maintained at a certain level when entered the third instar. The sclerotized area and the average optical density of the cephalopharyngeal skeleton were increased with the increase of ADH till the end of the third instar. The ADH that the sclerotized area and the average optical density reached the maximum was obviously higher than the one that the area of the cephalopharyngeal skeleton reached the maximum. Conclusion Morphine hydrochloride can obviously change the growth ADH of larvae of Parasarcophaga crassipalpis under natural conditions. The sclerotized areas and the average optical densities of the pharyngeal sclerite of the larvae show a good linearity with ADH till the end of the third instar, which can deduce PMI when combined together.