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Polystyrene microplastics (PS-MPs) are potentially harmful to marine organisms, especially during the early developmental stages, although the underlying mechanism remains unclear. The present study evaluated the growth and morphological characteristics of marine medaka <i>Oryzias melastigma</i> (McClelland, 1839) embryos exposed to PS-MP. PS-MPs of three different sizes (0.05, 0.5, and 6.0 μm with a concentration of 10⁶ particles/L) were subjected to waterborne exposure for 19 d. The hatching time and rate of embryos exposed to 0.5 and 6.0 μm PS-MPs were significantly lower than those of the control, while no significant difference was observed in the 0.05 μm treatment. No significant differences were observed in the mortality rate of the embryos, embryo diameter, and relevant gene expression levels, including <i>il6</i>, <i>il8</i>, <i>il-1β</i>, <i>jak</i>, <i>stat-3</i>, <i>nf-κb</i>, <i>hif-1α</i>, <i>epo</i>, <i>cyp1a1</i>, <i>ahr</i>, <i>sod</i>, <i>cat</i>, and <i>gpx</i>, but with the exception of <i>vtg</i>. Fluorescent PS-MPs were found on the embryo surfaces when the embryos were exposed to 0.5 and 6.0 μm PS-MPs, but no signals were detected inside embryos using confocal microscopy. Therefore, the results indicate that PS-MPs having a diameter of 6.0 μm can only attach to the surface or villus of embryos and not enter the embryos through the membrane pores, whereas PS-MPs with diameters of 0.05 and 0.5 μm cannot enter the embryos.