Abstract: The phylogenetic relation and taxomomic sta tus between genusChloromonas and genusChlamydomonas are uncertain in phycological field, since that the microstructure of them are much similar and bo th are unice llar groups with two equal-lengthed flagellas, the difference is just in that, theChlamydomonas species have more than one pyrenoid on the chloroplast whi le theChloromonas species have none ever since being reported. Therefore, as to the unicellar flagellated groups, the character of having a pyrenoid or not is of great significance in taxonomy. Whereas, many scholars have doubted the s tability of this character of not having pyrenoid and that is why the argument is still in pendency for long years. The recent research forChloromonas species are mostly carried out on micros copic levels, but not in the fields of ultrastructure, life history and molecula r phylogenetics. In this article we have compared the ultrastructure and microst ructure of a newly reported alga ofChloromonas sinica by H. J. Hu et C. T. Yin (2002) to that ofChlamydomonas species, finding that they are in highly similarity except for thatChloromonas sinica does have no pyrenoid in its whole life history, which starts from the period of spores. The truth is proved by the ultrastructure evidence. What’s more, we have also study the Chloromonas sinica by its flagellar structure, character of cell wall and chloroplast fi gures, knowing that the pattern of fibrovascular structure in the flagella trans ition region ofChloromonas sinica is much the same with that ofChlamydom onas species, and it is also the same for the young cell wall character in Chloromonas sinica, being compared to that of sporangium wall in theChlamyd o monas species. And there are some special differed characters betweenChloro monas sinica according to otherChloromonas species in the observation: 1) it has no colour and leads a heterotrophic life while cultured without lights s upply. Only under illumination can the chloroplast develop normally and the cells turn green with bigger irregular discoid chloroplasts, while the colorless cells are having smaller round ones in contrast, which still can be seen clearly in the microscopy. Besides these, the thylakoids are very rear and short in the co lourless cells. While in mature cells of normal condition the thylakoids are full of the whole chloroplast, which also possess 1 or more starch grains. 2) the c olourless cell wall is thicker than normal, being of 3 layers; the green ones’ c ell wall is very thin and have less layers or even just have one layer. However, this difference in cell wall being connected to its nourish pattern or not need s to go deep into in the future. Though molecular phylogenetics research of genusChloromonas and genusChl amydomonas shows that they are polyphylogentic, plenty studies are needed to do to confirm if Chloromonas is a subgenus ofChlamydomonas, or a genus of Chlamydomonas. And now before getting enough evidence, it is more reasonab le to take theChloromonas as a seperate genus from the genus Chlamydomonas.