Objective: The purpose of this study was to see the changes in muscle activity of the upper limb in persons using a smartphone. Design: Cross-sectional study. Methods: An experiment was conducted to target 15 right-handed university students. Experiments were carried out for students using cell phones for more than a year. In this study, experiments were carried out with one-handed and both handed operation of smartphone use in a sitting position, the same parameters with smartphone use in a standing position. The experiments were carried out by having the subjects write a text message in Korean on the smartphone for 3 minutes repeated 3 times with a rest period of 10 seconds given between each 3 minute period. Electromyography (EMG) was used to record the muscle activity of the upper trapezius (UT), extensor carpi radialis (ECR), and abductor pollicis (AP) during phone operation. Results: The muscle activity of the AP and ECR were significantly higher during single handed compared to double handed in both sitting and standing position (p<0.05). The muscle activity of the ECR was significantly higher in standing position compared to sitting position with double handed use of the smartphone (p<0.05). UT muscle activity of the right has been activated more than twice compared to the left UT in a sitting position (p<0.05). And UT muscle activity on the right has been activated more than five times compared to the left in a standing position (p<0.05). Conclusions: Using smartphone with double hand is useful for the prevention of musculoskeletal disorders.