Pyrenoids found in green algae Order Ulvales have taxonomical significance.Their morphology,microstructure,and the location and number in the cells have been widely used as identification criteria. As these traits can be affected by environmental conditions,it is necessary to look for other different characteristics.Here pyrenoids in several algae species collected along the Qingdao coast were studied for such purpose. The morphology and distribution of pyrenoids in the cells,as well as their change in different parts of the frond and at different times were studied with light microscope.The results are as follows.Pyrenoids appeared to be spherical or oval in observed Ulvales’algae cells.They were embedded in chloroplasts with the number of 1-3 per cell on average.The number of pyrenoids for Ulva pertusa could change drastically with continuing culture and could reach up to 10-20 per cell in late-growth stages.Generally, the pyrenoids in the marginal part were more observable than those in the central part for Ulva pertusa. In addition, the basal part of frond had more distinct pyrenoids with an average diameter of 2.42 μm for Enteromorpha linza. For some species the pyrenoids were difficult to be observed while dark treatment made them more distinguishable.The apoptosis rate of pyrenoids in different species varied under continuing dark treatment, which could be used to identify different species. Pyrenoids disappeared faster in E. linza than those in E.intestinalis. Pyrenoids found in green algae Order Ulvales have taxonomical significance.Their morphology, microstructure, and the location and number in the cells have been widely used as identification criteria. As these traits can be affected by environmental conditions, it is necessary to look for other different The morphology and distribution of pyrenoids in the cells, as well as their change in different parts of the frond and at different times were studied with light microscope. The results are as follows. Pyrenees were to be spherical or oval in observed Ulvales’algae cells. They were embedded in chloroplasts with the number of 1-3 per cell on average. The number of pyrenoids for Ulva pertusa could change drastically with continuing culture and could reach up to 10-20 per cell in late-growth stages. Normal, the pyrenoids in the marginal part were more observable than thos e in the central part for Ulva pertusa. In addition, the basal part of frond had more distinct pyrenoids with an average diameter of 2.42 μm for Enteromorpha linza. For some species the pyrenoids were difficult to be observed while dark treatment made them more distinguishable. The apoptosis rate of pyrenoids in different species varied under continuing dark treatment, which could be used to identify different species. Pyrenoids disappeared faster in E. linza than those in E. intestinalis.