Background: Internet addiction disorder (IAD) is rapidly becoming a prevalent mental health concern around the world. The neurobiological underpinnings of IAD should be studied to unravel the potential heterogeneity. This study was set to investigate the error-monitoring ability in IAD subjects. Methods: Fifteen IAD subjects and 15 healthy controls (HC) participated in this study. Participants were asked to perform a fast Stroop task that may show error responses. Behavioral and neurobiological results in relation to error responses were compared between IAD subjects and HC. Results: Compared to HC, IAD subjects showed increased activation in the anterior cingulate cortex (ACC) and decreased activation in the orbitofrontal cortex following error responses. Significant correlation was found between ACC activation and the Internet addiction test scores. Conclusions: IAD subjects show an impaired error-monitoring ability compared to HC, which can be detected by the hyperactivation in ACC in error responses.