The present study aimed to investigate whether the evaluative processing of action feedback is influenced by the cardiac cycle. To this aim, we examined the influence of the cardiac cycle on an event-related brain potential (ERP) effect that indexed the evaluative processing of action feedback [i.e., an ERP effect that interpreted as a feedback-related negativity (FRN) elicited by "bad" action feedback or a reward positivity (RewP) elicited by "good" action feedback]. Participants performed a gambling task in which they chose one of two stimuli and received an action feedback that indicated monetary gain or loss. Each trial was retrospectively classified into systole and diastole trials; systole trials were defined as those where monetary outcomes were presented during systole, and the diastole trials as those where monetary outcomes were presented during diastole. The results showed that the FRN/RewP was larger for the systole trials than for the diastole trials, which was due to the modulation of gain rather than loss ERPs. These results provide evidence that the natural fluctuation of cardiac afferent signals can modulate the evaluative processing of feedback.