This study aimed to investigate the effect of smartphone size and use on fatigue of the extensor pollicis longus and abductor digiti minimi, and digiti minimi deformity. The 40 people assigned to the experimental group spent at least 6 hours using the smartphone and used the digiti minimi for support. The control group spent at least 3 hours using the smartphone, however did not use digiti mini for support. Questionnaires were used to measure smartphone usage, daily use duration, and method of holding the smartphone device. X-rays of distal interphalangeal joint angle deformity of the digiti minimi were assessed. Surface electromyography was used to analyze muscle fatigue. The angle deformity was significantly different between the experimental group and the control group, and fatigue of the extensor pollicis longus and abductor digiti minimi of the large smartphone users were high. The muscle fatigue was reduced when the smartphone size fit well with the user's hand. This study suggests that the size of the smartphone influence the fatigue of the hand muscles.