The marine phytoplankton diatom Thalassiosira species are described from the Korean coastal waters, based onobservations of Fine structutre of its frustules. As a fourth series, this study deals with 5 Thlassiosira species (T. decip-iens, T. ecgentrica, T. mala, T. nodulolinmta, and T. proschinae). The most striking features of these species are also theform and position of two processes on frustules. There is a central strutted process on valve face except T. nodvlolin-eata, which has a modified one on its center. The extemal tubes of marginal strutted processes are divided twoforms: "pile" form in T. decipiens and T. proschkinae, "short straw" form in T. eccentrica and T. nodulolineata. T. maladoes not present the extemal tubes in valve margin. The labiate processes are located in valve margin except T.proschkinae, which has it on valve face. Of theae, thrce species (T. mala, T. nodulineata at a final contentration of 5%and T. proschkinae were newly recorded in the coastal waters of Korea.