Abstract: In order to establish the extraction method of microplastics in marine zooplankton, digestion efficiencies of organisms, recovery rates of microplastics, and changes of microplastics weight and infrared absorption spectra of microplastics before and after digestion were studied by using nitric acid (HNO₃) as the digestion reagent in this study. Furthermore, the effects of nitric acid concentration, digestion temperature and digestion time on the four different digestion methods were investigated. The results showed that the digestion efficiencies by using 14.4 M HNO₃ were significantly higher than that by using 1.4 M HNO₃. In addition, the weight changes of low-density polyethylene (LDPE), polypropylene (PP) and polyvinyl chloride (PVC) were less than 1.0% before and after digestion, and the recovery rates of LDPE microspheres, LDPE lines and polystyrene (PS) microspheres were greater than 85% by using 14.4 M HNO₃. Furthermore, the infrared absorption spectra for 5 types of polymers showed no significant changes before and after the treatments with 14.4 M HNO₃. In conclusion, the digestion method by using 14.4 M HNO₃ as the digestion reagent for 96 h at room temperature was recommended for the extraction of microplastics in marine zooplankton in this study. The method has some advantages including easy operation, mild reaction, relatively high biological digestion efficiency (92.3% ± 0.9%, n=3) and microplastics recovery rates (60.7%～95.5%). Therefore, this method was applied to extract microplastics in zooplankton sampled from the western sea of Dalian. The results showed that the abundance of microplastics ingested by zooplankton was 2.23 particles/m³, with two types of polymers (PET and rayon) detected. The shape of microplastics detected in zooplankton was fiber and the size of detected microplastics ranged from 0.20 to 1.67 mm. This study would provide scientific information for the standardization of monitoring methods about microplastics in marine organisms.