The domesticated horseshoe crabs (Tachypleus tridentatus) were collected from outdoor habitats and used on this study. In 2010, 10 males and 10 females were randomly picked and each was repeatedly drawn blood 30 mL every month. In 2011, 20 horseshoe crabs were randomly picked every month. Sampled horseshoe crab was drawn blood 30 ml each and had no extra blood drowning for the rest of months. The blood samples were mixed with anticoagulant and fixed by 10 times of formalin. The hemocytes were counted by the blood cell counter and used for analysis of monthly changes. Results showed that the numbers of hemocyte were correlated with sex and seasons. The means of number of hemocyte for male horseshoe crabs (20,445/µl and 24,191/µl, sampled in 2010 and 2011, respectively) were more than for those of female (17,865/µl and 20,067/µl, sampled in 2010 and 2011, respectively). The blood samples collected in summer and autumn had more hemocytes (23,334/µl and 25,141/µl, respectively) than other seasons. The mean of hemocytes sampled from May to September (25,051/µL) was higher than rest periods of the year. Considering to have more hemocytes in blood, the applicable period to harvest horseshoe crab blood for making Tachypleus hemocytes lysate could be from May to September.