Abstract: The effects of egg size on fertilization, fecundity, embryonic and larval development as well as offspring performance were compared between two closely related species of tropical sea urchins, Echinometra sp. A (Ea) and Echinometra sp. C (Ec) through laboratory experiments. Ec had significantly larger gametes in terms of egg diameter, egg volume and sperm head length and produced significantly larger but fewer eggs with higher fertilization rate than Ea. Developmental time from fertilization to the formation of echinus rudiment, just prior to the metamorphic competence of Ea were significantly longer than Ec, suggesting that increased allocation to energy reserves in larger Ec eggs reduced the development period as compared with the small Ea eggs. Consequently, smaller eggs of Ea produced smaller larvae than Ec throughout the larval period, due to the small amount of maternal investment as stored nutrients. As Ea and Ec have diverged from their ancestral species, the differences of the above traits related to egg sizes transcended species differences between Ea and Ec.