This study investigated effects of various concentrations of garlic extract (0, 2, 4, 8, 16, and 32 mg/L) on hatching rate, growth and survival of larvae and juveniles, metamorphosis, and disease prevention for Manila clam Ruditapes philippinarum. The results showed that the hatching rate decreased with increasing concentrations of garlic extract. A garlic extract concentration of 16 mg/L resulted in delayed embryo development, and 32 mg/L resulted in hatching failure of the embryos. During the planktonic stage, larval growth was depressed by garlic extract; larval survival and metamorphosis increased and then decreased as garlic extract concentrations increased. A garlic extract concentration of 16 mg/L appeared to be optimal for larval metamorphosis and survival. The metamorphosis size decreased with increasing garlic extract concentrations. During the indoor rearing period, the growth and survival of juveniles also increased and then decreased as garlic extract concentrations increased, with 8 mg/L being the optimum concentration. Results from the present study demonstrated that garlic extract concentrations of 8-10 mg/L could prevent diseases in the hatchery.