In this paper, the changes of facial expression during games was discussed, from the standpoint of behaviorism, using averaged facial electromyogram (AfEMG).<BR>In the experiment 1, subjects played a number selection game of man-machine interaction. When the subject's number was greater than computer's number, the subject was winner and gained money. When subject's number was smaller than computer's number, the subject was loser and lost money. The facial EMG was lateral frontalis, corrugator, and zygomaticus muscle regions on the right side of the face. The AfEMGs were analyzed the moment of subject's winning or losing to 1024 ms. There was no significant differences between AfEMG of the winning and losing cases. But there were clear differences bettween AfEMG of the winning and losing intra-personally.<BR>The experiment 2 was number selecting game without computer interaction. This experiment was designed by the multiple-baseline design. In the phase A, the subject selection a number from 0 to 4. In the phase B, when the subject's number was 0, the subject lost money. When the subject's number was 4, the subject gained money. When the subject's number was 1, 2, or, 3, these cases were cancelled. In the phase C, when the subject's number was 0 or 1, subject lost money. When the subject's number was 3 or 4, subject gained money. When the subject's number was 2, this case was cancelled. Facial EMGs was taken from the corrugator, zygomaticus muscle regions and posterior auricular muscle on the right side of the face. The method of AfEMG analysis was the same with experiment 1. The results of experiment 2 indicated that the activity of the zygomatic AfEMG increased at about 1000ms when the subject won.