Study on the ultrastructure of macro-algae is very important part in the basic and application study of algae. The ultrastructure of the zoospore, gamete, zygocyte, zoosporangia of M. latissimum was studied in this paper. The ultrastructure of Monstroma's cells closely correlated with their physiology function in the life-history. For example, the number of starch grains was changing in different life-history stage, the adherent function of mucilage occurred in early stage of zygote, the thick wall of sporangium was useful to resist bad environment. In the classification of the Monostroma complex, the ultrastructure and number of pyrenoids in the chloroplast were also taken as one of the taxonomic characteristics which are important supplement to conventional life-history and other morphology distinguishing characteristics. In every stage of life-history of M. latissimum there was one pyrenoid in each cell, the pyrenoid matrix was traversed by 2 or rarely 3 curved bounds of thylakoids. There were 3-4 or more pieces of starch plates covered the surface of the pyrenoid matrix. According to this pyrenoid ultrastructural characteristics, M. latissimum could be distinguished from other species of Monostroma.