Abstract: In the course of our investigation into the distribution of East sea algae, we have found, fry addition to Porphyra dentata Kjellm. and Porphyra suborbiculata Kjellm. previously described by famous scientists, several species of porphyra never described before. In this paper, only one of them, Porphyra dentimarginata, is reported upon. Charecteristics: Its blades are composed of monostromatic cells; Its form is generally round, elliptic, or like flowers; its colour is olive green or somewhat brown. Measures are: 2–14 cm in height, 2–9 cm in breadth, and 76–120 μ in thickness. Its base is discoid in shape. Its stipe is obscure; Its marginal portion consists of 2–6, sometimes more, rows of degenerated cells which are light in colour; when it is young, they are imperceptible, but when sexual cells are mature, they fall off the blade. Their outside is furnished with jagged teeth which are formed of 1–2 or at most 5 cells, sharp or blunt in form. Its vegetative cells are subrectangular with roundish corner in surface view , and sometimes they are 16–28 μ broad, 36–48 μ high in sectional view with a single stellate chromatophore . Plants heterothallic, spermatangia situated just inside the degenerated marginal portion, consists of 128 spermatia, and its mode of division is ♂ A4B4C8. Sporocarps scattered in the inside of the degenerated marginal portion, composed of 32 or 16 carpospores and its mode of division is ♀ A2B4C4 or A2B2C4 . HABIT and GROWTH: Porphyra dentimarginata grows on rocks which are near the middle of the littoral region of sea. Its growing season lasts always from the winter of first year to March of the next year. After April, on account of the highly rising temperature, it gradually vanishes. Systematic position: According to the main characteristic of Porphyra dentimar- ginata-marginal portion with jagged teeth and degenerated cells,it belongs to the section marginata of genus Euporphyra. We recognize in it a form of progress, that is, an evolution from section Edentata to section Marginata.