Municipal wastewater treatment plants (WWTPs) are a central point of collection of plastic particles from households and industry and for their re-distribution into the environment. The level of microplastics (MPs) in wastewater treatment plants (WWTPs) has been well evaluated by the particle number, while the mass concentration of MPs and especially nanoplastics (NPs) remains unclear. In this study, pyrolysis gas chromatography–mass spectrometry was used to determine the mass concentrations of MPs and NPs with different size ranges (10 nm – 1 µm and >1 µm) across the whole treatment schemes in three WWTPs. Results showed that the total plastic content in the WWTPs raw influent samples was between 840 and 3116 µg/L, resulting in an inflow of between about 2.1 and 196.4 kg/day of the total measured plastics. Overall, >99 % by mass of the plastics entering the three WWTPs from the raw influent was removed during the pre-treatment stages, presumably ending up in the sewage sludge, which means emissions (via treated effluent) from the treatment plants are low. Overall, this study investigated the mass concentration of MPs and NPs with a wide size range of 10 nm – 1 µm and >1µm in wastewater, which provided valuable information regarding the pollution level and distribution characteristics of MPs, especially NPs, in WWTPs.