Many researchers are concerned about the effects of video games on learning, especially the games with virtual opponents; therefore, the main purpose of this paper is to investigate the possible impacts. The authors designed Forest Defense, a digital game for practicing mental arithmetic, with two modes: training and challenge modes. In the challenge mode, students have to face virtual opponents which have not been shown in the training mode. A preliminary experiment was conducted to examine the effects of the challenge mode in Forest Defense. The results showed that virtual opponents did not influence the accuracy of the high-ability students, but significantly decreased the accuracy of the low-ability students. The results also suggested that if practicing more, the low-ability students were more able to face virtual opponents.