Nutrient removal from digested piggery wastewater (DPW) using microalgae is of increasing interest as a secondary treatment prior to discharge to avoid environmental contamination. In this study, we investigated the main mechanisms involved in ammonium removal from DPW by Oedogonium sp. using the 15N mass balance approach with a focus on the relationship between algal growth and ammonium removal and the dominant ammonium removal pathway. 96.2% ammonium removal and 0.04–0.15 specific growth rate of Oedogonium sp. were observed in the diluted autoclaved DPW during the incubation period, while 94.1 % ammonium removal and -0.14–0.13 specific growth rate of Oedogonium sp. were observed in the diluted raw DPW. Aeration provided a significant benefit to ammonium removal via the stripping effect, which was favored by the high pH in the experimental conditions. Isotope mass balance analysis indicated that bacteria existing in the initial DPW had little effect on ammonium removal in the experiment. And algal uptake and gaseous loss were the dominant pathways for NH4-N removal from the diluted DPW using Oedogonium sp. cultures, accounting for 40.97% and 32.59% of the total 15N amount, respectively. Regression and path analyses between NH4-N removal and its main influencing factors indicated that to improve NH4-N removal efficiency, the levels of Oedogonium sp. and DO should be increased under weakly alkaline conditions.