This report investigates attribution processes using 68 students in a cooperation-noncooperation modified prisoner's dilemma game. The attribution processes of the subjects were explored in a 2 × 2 factorial design (subject's high or low cooperation and target's high or low cooperation). Experimental results showed that subjects attributed the target's high cooperation significantly less to an internal cause when the subject's own cooperation was high than when it was low. There was no significant difference in the subject's ratings of the internal attribution for the target's low cooperation between the subject's cooperation conditions. When subjects attributed the less to the internal cause, the subjects had a tendency to attribute the cooperation to subject's own cooperation. These results confirmed that an active observer in a social interaction perceives the causes of another person's behavior dependent on observer's own behavior toward the other. Adjective ratings of the target's dispositions gave further evidence.