Abstract: To study the toxic effects of nanoplastics on marine microalgae, polystyrene (PS) nanoplastics with the particle size of 0.1 μm in different concentrations (0.1 mg/L, 1 mg/L and 10 mg/L) were selected as the pollutants to investigate their toxic effects on Skeletonema costatum in this paper. The results showed that the growth of S. costatum cells was significantly inhibited and the level of intracellular reactive oxygen species (ROS) increased under the treatments of PS nanoplastics. When the concentration of PS nanoplastics was 10 mg/L, the contents of malondialdehyde (MDA) increased in the cells. The intracellular superoxide dismutase (SOD) activities in the cells also increased when the concentrations of PS nanoplastics were 0.1 mg/L and 1 mg/L, but the intracellular SOD activity increased first and then decreased when the concentration of PS nanoplastics was 10 mg/L. The activities of intracellular catalase (CAT) and the rates of cell apoptosis increased with the extension of incubation time. This study can provide help to scientifically assess the toxic effects of PS nanoplastic pollution on S. costatum.