Background and Objectives: With the pervasive use of digital devices, especially smartphones, concerns about their harmful effects on vision has increased. The aim of the present study was to evaluate the effect of long-term activity with a smartphone on the accuracy of lag of accommodation of two eyes. Methods: In this cross-sectional study, 27(16 females and 11 males) students and staff (23 students and 4 staff) of the Iran University of Medical Science with a Mean±SD age of 25.29±4.72 years, with a visual acuity of 10/10, and emmetropic refraction with the available sampling method were selected. At first, the participants used a smartphone for an hour, and then, the lag of accommodation of two eyes was assessed with the monocular estimate method. Results: lag of accommodation of the right eye (P=0.001) and left eye (P=0.001) both statically increased significantly after one hour of smartphone use. However, no significant difference was found between the lag of accommodation of two eyes after near work with a smartphone (P=0.265). Conclusion: The present study confirmed the relationship between long-term near work with a smartphone and increased lag of accommodation.