The cytology of the fertilization and early embryonic development in Ruditapes philippinarum were consecutively observed by the fluorescent microscope with HOECHST 33258 stained. The results showed that the mature eggs were at metaphase of meiosis Ⅰ. Sperms bound to the eggs and began the acrosomal reaction quickly after mix of sperms and eggs . With the activation of the dispersed sperms , the eggs released their first and second polar bodies and finished their meiotic division. The early male and female pronuclei were formed when the second polar body was released. The two pronuclei were expanded and closed to each other , finally associated with each other after the chromosomes formed in the central of the egg respectively. The development is quick in the early embryogenetic stage. There are a few of polyspermy phenomena in the fertilization process in Ruditapes philippinarum.