Microplastics in aquatic environment can possibly enter into the aquatic organism, so this study aims to identify the microplastic concentrations in water, sediment, and milkfish. The samples are taken in downstream Citarum river at Muara Gembong. Water samples were taken using a manta trawl pulled by a boat while sediment and milkfish samples were taken using the grab sampling method. Organic removal and cleaning was conducted by using the Fenton Oxidation method which use Fe and H₂O₂. The average of microplastic concentrations in river water, seawater ponds, mixed water, and seawater consecutively are 0.0574 ± 0.025 particles/m³, 3.000 ± 2.645 particles/L, 0.666 ± 0.577 particles/L and 1.333 ± 1.155 particles/L. The average of microplastic concentrations in sediment sea water pond, sediment mixed water pond and sea sediment samples consecutively are 3.666 ± 0.577 particles/20g, 2.667 ± 1.527 particles/20g, 2.333 ± 0.577 particles/20g and 0.667 ± 1.154 particle/20g. The average of microplastic concentrations in the gut and gills of milkfish in sea ponds and mixed ponds consecutively are 2.333 ± 2.266 particles/fish and 2.222 ± 3.768 particles/fish. The average of microplastic concentrations in milkfish tissues in the sea and mixed water ponds are 1.333 ± 1.000 particles/fish and 1.111 ± 1.167 particles/fish.