The pyrenoid ultrastructure, gold immunolocalization of Rubisco and Rubisco activase in chloroplast of Enteromorpha clathrata were studied in this paper. E. clathrata is a marine macroalga with rapid growth and mutiple pyreno ids. The ultrastructure studies showed that there were 1 - 2 pyrenoids with similar shape and component in the cells of E. clathrata. The pyrenoids were embedded by starch sheath. There was a portrait channel formed by a thylakoid through pyrenoid. Sometimes the portrait channel was enlarged in the middle of pyrenoid and was connected with the stroma of chloroplast at both channel ends. The Western blotting with antibody raised against Rubisco large subunit from Chlorella protothecoides showed there was only one protein band in the Western blotting map. The SDS-PAGE and Western blotting showed the molecular weight of Rubisco large subunit of E. clathrata was about 55kDa. Gold immunolocalization showed the gold particles with antibodies raised against the large subunit of Rubisco heavily concentrated the pyrenoid ( 71. 86% ) and starch sheath ( 27. 94%) , both totalling 99. 8% by area calculation, whereas the thylakoid region of the chloroplast was lightly labeled ( 0. 2% ) . Rubisco act ivases also were mainly distributed in the pyrenoid and starch sheath. All the results demonstrated pyrenoid and starch sheath were closely related with photosynthetic function. Furthermore, the result s of initial and total activity of Rubisco in E. clathrata determination of Rubisco showed its active ratio of Rubisco was as high as 77. 62% .