Abstract: This is a study which researches on the methods of evaluating the vitality and the quality of the postlarvae of Penaeus monodon by observing and fastness-testing. The results show thathealthy postlarvae had the characters of phototaxis, gregarious instinct, the body-surface of the postlarvae was clean, the tissue was transparent and the stomach and intestine were full. The appropriate time for postlarvae living without water was 15 minute. After that, the vitality of the healthy postlarvae was resumed quickly while the sick ones were dead and narcose. The survival rate of postlarvae was reduced when the time of starvation was postponed. The healthy postlarvae could sustain about 30 minutes in 100~200 LL-1 Formalin and the survival rate achieves 100%. However, the survival rate was decreased significantly with increasing of the Formalin concentration and time. With the increase of the sudden change of salinity, the activity of the postlarvae decreased. The appropriate range of salinity of postlarvae was 20~30, but its only sustained 1 hour in fresh water. It could be avoid the disease that the virus were detected of the postlarvae.