Ubiquitous occurrence of fine plastic particles including microplastics (MPs) and nanoplastics (NPs) in the environment has been causing increasing concerns, however, their degradation in nature has not been well evaluated yet. One of the reasons is that the difficulty in their detection and quantification. Here, we used 14C-radioisotope tracer technology to overcome this limit. We chemically synthesized 14C-labelled polystyrene (14C-PS) in both nanometer and micrometer sizes and studied photo-degradation and fungal degradation of PS. The results showed that UV-irradiation could transform PS and mineralized nanoplastics both in air and water, but the presence of water significantly increased the mineralization and generated by-products with small molecular weight, while irradiation in air increased the molecular weight of the PS nanoplastics, suggesting that water molecules were involved in photo-transformation of plastics in the environment. Additionally, in the presence of water, more soluble transformation products were produced, which were mineralizable in natural water. In the case of fungal degradation of PS, high density of Penicillium variabile biomass in liquid medium without additional carbon substrate could mineralize PS, and the PS particles with a lower molecular weight had a higher mineralization rate. Chemical pretreatment of PS particles with ozone O 3 strongly enhanced the mineralization of PS. Ozonation generated carbonyl groups on PS surface and the amount of the carbonyl groups decreased after incubation of the PS with the fungus. The data suggest that ozonation pretreatment could be a potential approach for degradation of PS waste and remediation of PS-contaminated sites. Our studies provided direct evidence of abiotic degradation of PS nanoplastics in aqueous environment and of biotic degradation of PS with chemical pretreatment.