Polystyrene nanoparticles (PS NPs), originated from breakdown of large plastic wastes, have already caused much concern for their environmental risks on health. This current study was aimed to reveal the toxicological mechanism of PS NPs on developing zebrafish and macrophage cells. To fulfill this purpose, 42 nm PS NPs were exposed to the early development stage of zebrafish for 5 days, the decreased heart rate and locomotor activity of zebrafish larvae were observed. The fluorescent PS NPs were used to precisely assess the accumulation of PS NPs in zebrafish larvae, and the results indicated that PS NPs not only accumulated in digestive system, but also infiltrated into the liver. More importantly, the transcriptomic analysis revealed that a total of 356 genes were differentially expressed and the KEGG class map showed significant differences in the MAPK pathway upon PS NPs treatment. Meanwhile, the induction of oxidative stress and inflammation were also observed in zebrafish larvae. Furthermore, PS NPs also induced oxidative damage and inflammatory response in RAW 264.7 cells, which activated p38 MAPK signal pathway and finally induced cell apoptosis. Our study provides a new understanding of MAPK signaling pathway involved in toxicity mechanism. Display omitted •Nanoplastics posed great potential risks to aquatic organism and mammalian cell.•Nanoplastics induced significant alterations in MAPK pathway in zebrafish larvae.•The induction of oxidative stress and inflammation were correlated with p38 MAPK pathway.•Nanoplastics activated p38 MAPK pathway and finally induced apoptosis.