Growing evidence shows that enhanced reactive oxygen species (ROS) production is an important contributor to obesity and its co-morbidities, but the functional link between ROS and obesity remains elusive. In this study we used the model animal Caenorhabditis elegans to explore the role of ROS in obesity. Initially, when ROS production was enhanced by treatment with low concentration of paraquat or juglone, both abnormal high fat accumulation and fatty acid composition were observed in wild type worms. We found that the abnormal fat accumulation was associated with increased expression of fat-5, which encodes an isoform of stearoyl-CoA synthetase, and which is regulated by daf-16 encoding the forkhead transcription factor and being activated by downregulation daf-2. When mutant daf-16 worms were used, the abnormal fat accumulation induced by ROS was suppressed. Collectively, we demonstrate that enhanced ROS production can lead to excessive fat accumulation and the change of fatty acid composition. This abnormal phenomenon at least in part depends on the daf-16 pathway by which fat-5 was regulated. The results point towards a role of ROS in obesity in the context of important conserved signaling pathway, thereby guide further studies and future therapeutic interventions.