In order to comprehensively investigate the effect of size (mm-, μm-, and nm-size) on the polyethylene microplastic (PE-MP) induced changes in soil chemical, physical, and microbial properties, a 180-day indoor soil incubation experiment was conducted here. The results demonstrated that: after the 180-day incubation period, all the three sizes of PE-MPs significantly improved the stability of soil aggregates, especially for the μm-size PE-MP (μMP); the addition of PE-MP, regardless its particle sizes, significantly reduced the NH₄₊^-N content, and the reductions were more significant in the mm-size PE-MP (mMP) and μMP treatments; the NO_3?^-N content significantly decreased in the mMP and μMP treatments, while it changed insignificantly in the nm-size PE-MP (nMP) treatment; all the three PE-MPs significantly reduced the soil pH, and the maximum reduction was observed in the case of nMP; for the dissolved organic carbon (DOC) content, it was found to significantly increased in the mMP treatment, but dramatically decreased the nMP treatment. Lastly, the PE-MPs addition consistently increased the richness and diversity of bacterial community, while decreased the richness and diversity of fungal community; the mMP and μMP significantly reduced the relative abundance of microbial taxon associated with unstable carbon utilization (e.g., Proteobacteria), while the nMP significantly reduced the relative abundance of microbial taxon associated with relatively stable carbon utilization (e.g., Sordariomycetes) and nitrogen conversion (Myxococcota). Based on the above results, the potential effects of PE-MP-induced changes of soil properties on soil quality was further discussed.