Microplastics (MPs) are a class of emerging contaminants with diverse sizes. They influence the behavior of pollutants in the environment and cause harmful effects on organisms. To date, the size effects of MPs on the accumulation of organic pollutants by terrestrial invertebrates remain unclear. Here, we study the impacts and mechanisms of polystyrene MPs on the accumulation and elimination of phenanthrene in earthworms. Results showed that larger-size MPs (10 and 100 mu m) facilitated the accumulation of phenanthrene by earthworms in the first week, whereas 100 nm MPs inhibited the elimination of phenanthrene in earthworms afterwards. Higher genotoxicity to earthworms was observed for co-exposure of micron-size MPs and phenanthrene, and 10 mu m MPs were detected at the highest concentration and caused the most serious DNA damage to earthworm coelomocytes. Biomarkers and their mRNA gene expression levels suggested that larger-size MPs caused severer damage to earthworms, thus leading to increased accumulation of phenanthrene by earthworms at the beginning. Moreover, high-throughput 16S rRNA gene sequencing indicated that nano-size MPs significantly inhibited phenanthrene-degrading bacteria in earthworms, resulting in the highest residual concentration of phenanthrene. This study highlights the size effects of MPs and their impacts on the accumulation of organic pollutants by terrestrial organisms.