We investigated the processes of egg and larval developments for aquaculture technique development of seedling production fo the flat oyster, Ostrea denselamellosa. Teo flat oyster of larviparous type was different from the pacific oyser (ovivarous type) because their larvae (trochophore and prodissoconch larvae) in the gill released into the seawater. The process of egg development was observed by artificial fertilization at $25^{circ}C$, using a dissecting method. The sizes of Unfertilized eggs ranged from 80 to 90 $mu extrm{m}$ and fertilized eggs with globule-shape was 90-100 $mu extrm{m}$. The Polar body appeared after fertilization and egg cleavage began within 1 hour, reaching the blastula stage after 10 hours. The trochophore in the gill appeared 2-3 days after fertilization and grew to the prodissoconch larvae (130 140 $mu extrm{m}$) having a complete shell after 1-2 days. The shell of prodissoconch larvae grew to 205 220 $mu extrm{m}$ after 10 hours, and then they became umbo stage larvae showing oval in shape. The velum of umbo stage larvae was degenerated about 17-20 days after fertilization and grew into a pediveliger with a developed foot, at this time, the shell length size was 320 360 $mu extrm{m}$.