Previous research has revealed that interpersonal relationships and social comparisons play important roles in evaluating outcomes. To our knowledge, how interpersonal relationships influence the process of outcome evaluations in a social comparison context remains largely unclear. In the current study, participants engaged in a simple gambling task with an acquaintance or a stranger and received outcome feedback. Behavioral results showed that participants' satisfaction level was sensitive to the outcome of their fellow players when participants won. In this condition, the satisfaction level was greater when their fellow players won rather than lost. Moreover, the satisfaction level was greater when their friends won compared with when a stranger won. Event-related potential (ERP) results showed that when participants won, the feedback-related negativity (FRN) was more negative going for other's losses than for other's gains. Moreover, the FRN was also more negative going for a stranger's gains than a friend's gains. In contrast, in the self-loss condition, the FRN was more negative going for other's gains than for other's losses regardless of the type of interpersonal relationship. These FRN findings indicate that the experience of other's outcomes is sensitive to participants' own outcomes. Importantly, the interpersonal relationship only showed its influence when both the self and others received monetary gains. Finally, the P300 registered participants' attention resource allocation toward monetary gains for themselves and for others, which was unaffected by the interpersonal relationship. This work reveals that outcome evaluation in various social comparison contexts is sensitive to the difference in interpersonal relationship in its early stage, labeled by the FRN.