Morphology of 4 dinoflagellates species, Amphisolenia bidentata, A. inflata, A. thrinax and Triposolenia bicornis, which belong to the Dinophysiales, were studied by light and scanning electronmicroscope. The plankton samples for this study were collected from Jeju-Do, South Korea in February, April, September and November 2007. Three Amphisolenia species have large cell size ($125{sim}890{mu}m$) then T. bicornis ($45{sim}50{mu}m$). Cells of Amphisolenia and Triposolenia with unique morphological features were divided into the head, neck, anterior part of the body (APB), mid-body and posterior process. This is the first record of those four species from Korea.