The effects of finely-chopped straws and aqueous extracts from barely straw and rice straw on the growth of Phaeocystis globosa were observed. The roles of microorganism and possible adsorption of chopped straws in the inhibition were assessed. The morphological changes in algal cells induced by straw extract were observed by AFM. Both finely chopped straws and aqueous extracts of straws were showed to have inhibitory effect on Phaeocystis globosa. There were little difference in algistatic action between asepsis straw and rude straw, suggesting that some antialgal compounds from straws might be responsible for the inhibition, and that physical adsorption and microorganisms from straw had little effect on the inhibition. The total phenol levels and antialgal action of the extracts obtained under various conditions were different, but no direct relationship between phenol level and antialgal action were found, which indicated that the antialgal compound or algaecide other than phenol, existed in extracts. Morphological changes such as increase in cellular roughness, shrink of cell structure and rupture of cell membrane, etc. were observed by AFM in the algal cell treated with extracts. From the results, it was suggested that barely straw and rice straw had certain inhibitory effect on Phaeocystis globosa and some antialgal compounds besides phenol might be responsible for the inhibition. Damages of cell membrane might be cause of the death of the algal cell induced by straw extract.