The aim of this study is to understand the distribution of phytoplankton to assess the performance of a free water surface (FWS) constructed wetland based on the water quality index (WQI) evaluation. The constructed wetland consists of three main zone; forebay zone, macrophytes zone and micropool zone. The highest WQI value was obtained in the micropool zone with the range of 78.98-85.45 (82.71±3.35), while the lowest WQI value was obtained in the macrophytes zone with the range of 68.13-77.96 (73.25±3.42). The higher distribution of phytoplankton was obtained in the macrophytes zone due to the high nutrient concentrations available. The dry season showed higher concentration of the nutrient, ammoniacal nitrogen, compared to the wet season. The distribution of phytoplankton was high during the dry season compared to the wet season. Phytoplankton distribution is closely related to dissolve oxygen, water temperature and WQI value in the FWS constructed wetland.