Using electromyography (EMG) analysis and psychological rating, the usability of pen was evaluated. The experimental factors wewre the pen diameter (three levels:8mm, 11mm, and 13.8mm). The length of pen tip was fixed to 14mm. Surface EMG was recordes from extensor digitorum and flexxor digitorum superficialis. The EMG before and after a long-hour writing task was measured to evaluate the fatigue of forearm using % MVC (Mean Voluntary Contraction), mean power frequency (MPF) and psychological rating on usability. Concerning % MVC and MPF, the difference before and the experimental task were used for the evaluation. The evaluation value corresponded to subtraction of value after the experimental task from that before the experimental task. As a result of a one-way (pen diameter) ANOVA, no significant main effects of pen diameter were for both EMG evaluation measures. As for the psychological rating on usability, Kruscal-Wallis non-parametric test was carried out. The psychological rating on ease of grip revealed a significant main effect of pen diameter. A pen with a diameter of 11mm was found to have a significant higher rating score.