Abstract: The ultrastructure of flagellar apparatus and the development of theca in the marine quadriflagellate green alga Tetraselmis guangdongensis have been studied in detail. The four flagella emerge from an apical trough in theca and arrange in a zigzag ( "Z" ) row with their long axes nearly parallel. They are covered by two layers of scales. The striated fiber (transfiber)connects each outer basal body with the inner basal body of the opposite, mirror image pair. A complex system of four laminated oval dics(halfdesmosme, rhizanchora), microtubule and fibrous material anchor the flagellar apparatus and rhizoplast to plasma membrane and theca. A rhizoplast with two bundles cross microtubulous band linking to two near outer basal bodies and two rhizanchora. This character is different from other Tetraselmis.The development of theea begins in the endomembrane system, particularly the Golgi apparatus where fibrillar tufts and electron-dense particles are synthesized, modified and transported to the outside by vesicles derived from the Golgi apparatus. Here, the tufts and droplets are displaced at a predetermined place on the surface of protoplast, then surround it and assemble in several steps to coalesce the intact theca. This place is related to the position of the pyrenoid. It is suggested that an enzyme capable of agglutinating stellate particles is probably elaborated or liberated at the site of pyrenoid and secreted thence to the cell exterior Edge-growth assembly of the theca occurs here synchronously with cytoplasmic developments yield the charateristic anterior flagellar pit.