Smartphones have been the most popular electronic devices, especially among the adolescent. Smartphones are used for multi-purposes including communication, music, media, internet access, games, some applications, and professional fields. Using the smartphone frequently forces the users to adopt an awkward posture leading to an increased risk of musculoskeletal disorders and pain. Purpose To determine the effect of smartphone use on endurance of cervical flexor muscles. Methods: Sixty adolescents of both sexes ranging in age from 14 to 18 years were enrolled in this study. They were assigned randomly into two groups of equal number; group (A) used smartphone less than 4 hours/day and group (B) used smartphone more than 4 hours per/day. Participants of both groups using smartphones for at least 4 years. The pressure biofeedback was used to detect the endurance of deep cervical flexor muscles Result: Significant decrease was observed in endurance of deep cervical flexors muscles for both groups (A, B). high significant difference was observed in favor of group B when compared to group A.  (p < .05). Conclusion: the overuse of smartphones has a negative effect on endurance of deep cervical flexor muscles.