[Purpose] The purpose of this study was to examine the effects of the cervical flexion angle on muscle fatigue and pain in the cervical erector spinae and upper trapezius in normal adults in their 20s. [Subjects and Methods] The study's subjects were 14 normal adults. After sitting on a chair with their back against the wall, they held a smartphone with both hands for 10 minutes and fatigue and pain in the neck and shoulder muscles were measured at different cervical flexion angles (0°, 30°, and 50°). Electromyography was performed to analyze the muscle fatigue of the right upper trapezius, left upper trapezius, right cervical erector spinae, and left cervical erector spinae, and a Commander Algometer was used to measure pain. The cervical range of motion was used as an instrument to compare and analyze the cervical flexion angles. [Results] The study's results showed statistically significant differences in the muscle fatigue and pain of the right upper trapezius and left upper trapezius depending on the cervical flexion angle and a post-hoc test showed statistically significant lower levels of muscle fatigue and pain at 50° than at 0° or 30°. No statistically significant differences were found between the right cervical erector spinae and left cervical erector spinae. [Conclusion] The cervical flexion angle during smartphone use may influence the muscle fatigue and pain of the upper trapezius.