Abstract: In this paper, the toxic effects of 120# fuel oil water accommodated fractions (WAFs) on marine medaka (Oryzias melastigma) embryo were studied, with comparison of the sensitivities of embryos on different developmental stages.The marine medaka embryos of 6, 18 and 30 hpf (hour post-fertilization) were exposed to different concentrations of WAFs.The responses of embryos were observed and recorded at different time (12, 24, 48, 72 and 96 h after exposure), and the blue sac disease (BSD) index, hatching rate, heart rate and LC₅₀ were calculated separately.The results showed that WAFs exposure could cause obvious toxic effects, including malformation, decrease in hatching rate and heart rate, and increase in mortality, with significant time-dose effect relationships.The developmental endpoints showed a varying extent of sensitivity to WAFs among which the changes of BSD index and hatching rate were more significant.The sensitivity of embryos to 120# fuel oil gradually decreased with the development.The toxicity gradually decreased with the delay of the starting point of exposure.But the WAFs affected the heart rate mainly in the stage of cardiac development.