The embryonic development of the sea slugOnchidium sp. was observed and described at 3 temperature groups and the main factors affecting the veliger larva released were also examined in artificial conditions. Results showed： The embryonic development process of sea slug was similar to that of other Pulmonate species. The newly laid eggs were fertilized eggs with no division. They then entered into 7 stages such as the cleavage, blastula, gastrula, intracapsular trochophore, veliger in capsule, hatching or release, and the free-swimming veliger, At water temperatures of 26℃, 28℃ and 30℃, it took 300, 290, 288 hours respectively, of development from the zygote to the free-swimming veliger for the sea slug, The body sizes, shapes, interior structures and the metamorphosis of every period of larvae were also recorded under microscopic observation, It is found that the main factors that affected the release were water temperature and the stimulation of sea water, The ceasing of embryo development was first reported at the temperature of below 23℃.