This study investigated the effects of increasing mercury (Hg) concentration on early developmental stages of sea urchin, Echinomethra mathaei, as a bioindicator. The toxicity test was carried out after the gamete released induction and fertilization in six concentrations of mercury within the range of 4, 8, 16, 32, 64 and 128 µg/L. Embryos samples were incubated for 30 h in control and test solutions. After incubation, the percentage of developed 4-arm pluteus larvae was recorded in each group and embryonic abnormalities were studied by a microscope. Results of this study indicate that exposure of embryos to increasing mercury concentrations lead to abnormalities such as changes to shape and size of pluteus larval arms and also arrested development in early embryonic stages. Furthermore, embryos were analyzed to determine mercury absorption by cold vapour atomic absorption spectrometry method (AAS). The median effective concentration (EC50) value calculated for mercury was 17/42 µg/L.