Abstract: Microplastics (MPs), an emerging environmental pollutant, have drawn increasing public concerns for its detrimental environmental effects. To evaluate the effects of MPs with different sizes on feeding behavior and antioxidant system of freshwater organisms, three different sizes of polystyrene microplastics (i.e. 100 nm, 5 μm, 50 μm) were selected to investigate the accumulation and residence of MPs in Daphnia magna, and the influence of MPs on the filtration rate, feeding rate, and antioxidant system of Daphnia magna. The results showed that all MPs could be ingested by Daphnia magna, and the accumulation and residence of MPs with a size of 100 nm and 5 μm in Daphnia magna were higher than 50 μm. Moreover, MPs with a size of 50 μm were mainly adhered to the thorax appendages, while MPs with a size of 100 nm and 5 μm could be accumulated in the intestines of Daphnia magna. After exposure to the three MPs, the filtration rate and feeding rate of Daphnia magna decreased by 50.7%±9.5% and 39.2%±10.7% compared with the control group, respectively. Furthermore, the activity of superoxide dismutase and catalase, and the content of malondialdehyde in Daphnia magna increased significantly, and the oxidative system of Daphnia magna was damaged. Overall, the exposure of MPs can cause the damage to the feeding filter of Daphnia magna, block the digestive tract, inhibit the filtration rate and feeding rate, and ultimately lead to the damage of oxidative system Daphnia magna. Therefore, the exposure of MPs can lead to adverse effects on feeding behavior of Daphnia magna.