Abstract: Microplastics have been detected in various marine organisms and have caused significant toxic effects. Due to the limitations of detection and quantification methods, the dynamics of marine fish ingestion and egestion of microplastics are still not well understood. With the aim to overcome the difficulties of quantifying microplastics in organisms, especially for microplastics of small sizes and low concentrations, this study proposed two methods using fluorescence and C-14 isotope tracer. The advantages of these two methods in terms of detection limit, sensitivity, qualitative and quantitative aspects were compared. Moreover, these two methods were used to quantify PS microplastics (< 1 μm) ingested by marine medaka (Oryzias melastigma) and the influence of ingestion/egestion process on microplastics in seawater. The results showed that fluorescence labeling was suitable for mapping and quantification of high-concentration microplastics in organisms, while the C-14 isotope tracer was more sensitive to microplastics in low-concentrations or complex environmental matrices. Large ingestion was observed during the incubation experiments and reached a maximum amount at 24 h. The microplastics concentration in adult medaka ((246.8±38.1) mg/g in wet weight) was significantly higher than that of juvenile fish ((4.32±0.77) mg/g in wet weight). The distribution of microplastics in adult medaka were detected highest in the gut of marine medaka (99.9%), followed by 0.07% in the gill and 0.03% in the body, indicating food ingestion was the dominant way of microplastic accumulation. After 72 h of egestion, microplastics could still be detected in the gut of marine medaka ((1.29±0.52) mg/g), but no microplastics could be detected in the gill and body, indicating potential ecological risks for marine organisms. It is worth noting that the ingestion of microplastics by marine medaka could significantly change the exiting state of microplastics from dispersion into aggregation, which may change the environmental fate of microplastics in marine systems. The unknown outcomes caused by such alteration on behavior and ecological effects of microplastics deserve more attention.