Abstract: In this study, micro-sized particles polyethylene terephthalate (PET) and polystyrene (PS) were selected as the targeted pollutants, and the effects of PET and PS at different concentrations (50 mg/L, 100 mg/L and 200 mg/L) and particle size (100 μm, 175 μm and 250 μm) on the growth of microalgae Chaetoceros muelleri were investigated. The results showed that three particle sizes and concentrations of microplastics (PET and PS) all had inhibitory effects on the cell density of microalgae. The maximum growth inhibition ratio (IR) reached up to 37.3% and 32.5%, respectively, when the microplastics are at the concentration of 200 mg/L and the size of 100 μm. The inhibitory effect of microplastics to C. muelleri increased with the concentration of PET and PS, but no obvious effect caused by the particle size. As compared with the control group, PET slightly inhibited (P>0.05) the dry cell weight of C. muelleri, while PS had no inhibition on the cells. The inhibitory effect of microplastics on chlorophyll a (Chl a) is related to the type of microplastic and the length of exposure time. The PET inhibited while PS promoted the Chl a content of C. muelleri under the investigated conditions.s on chlorophyll a (Chl a) is related to the type of microplastic and the length of exposure time. The PET inhibited while PS promoted the Chl a content of C. muelleri under the investigated conditions.