Microplastics are identified as a great threat to marine environments. However, knowledge of their impacts on phytoplankton, especially for the diatoms is scarce. Herein, the effects of different polyvinyl chloride (PVC) microplastic concentrations and contact times (24, 48, 72 and 96 h) on the F-v/F-m and cell density of Phaeodactylum tricornutum (B255), Chaetoceros gracilis (B13) and Thalassiosira sp . (B280) were investigated to evaluate the toxic effects of micmplastics on marine diatoms. The effects of PVC microplastics on the morphology of the diatoms was observed by SEM. The order of sensitivity to 1 mu m PVC microplastics among three marine diatoms was B13 > B280 > B255, showing that the toxic effects varied with different microalgae species. Furthermore, the presence of a siliceous cell wall played a minimal role in protecting cells from the physical attack of PVC microplastics, with no significant difference from the common cell wall. PVC microplastics caused dosedependent adverse effects on three marine diatoms. High PVC concentrations (200 mg/L) reduced the chlorophyll content, inhibited F-v/F-m, and affected the photosynthesis of three marine diatoms. The PVC micmplastics adsorbed and caused physical damage on the structure of algal cells. Interactions between PVC microplastics and diatoms may be the probable reason for the negative effects of PVC on diatoms.