Purpose: This study aimed to examine changes in the range of motion (ROM) and cervical posture depending on the duration of using smartphone in sedentary asymptomatic adults. Subjects and Methods: The participants were randomly assigned into 2 groups: group A (those using smartphones for less than 4 h/day, n=200) and group B (those using smartphones for more than 4 h/day, n=200). The cervical posture was determined by measuring the craniovertebral angle (CVA) of all participants using the photographic method. Cervical flexion, extension, rotation (left and right), and side bending (left and right) range of motion (ROM) were measured by using an electromagnetic tracking device (MTD) mounted on a custom headpiece. Results: Significant differences were noted for cervical CVA and ROM between the groups (p < 0.05). Conclusion: The long-term effect of the smartphone overuse could negatively affect cervical posture and range of motion