Background: One of the most important threats for children's health status is being overweight and obesity, and related causes such as screen time prevalence. Prevalence of being overweight and obesity in children is associated with health risk consequences in adulthood. Objectives: The purpose of this study was to assess the prevalence of screen time and physical activity in overweight and obese students. Patients and Methods: In this cross sectional descriptive study we randomly selected 302 students, from all districts of Qazvin, who's relative body mass index (RBMIs) were above the 85th percentile. Their screen time and physical activity prevalence were assessed with two separate techniques (self-report and parent-report). Pearson correlation test and regression analysis were done to examine the association between RBMI, screen time and physical activity. Results: Mean screen time in boys was more than girls, in both overweight screen time-self report technique (ST-SRT: 1.93 ± 0.24 vs. 1.26 ± 0.44, ST-PRT: 3.4 ± 0.22 vs. 2.1 ± 0.15) and obese subjects (ST-SRT: 1.88 ± 0.31 vs. 1.37 ± 0.49, ST-PRT: 3.2 ± 0.32 vs. 2.3 ± 0.34) yet overweight subjects had less total screen time than obese individuals (P < 0.05). Pearson correlations with one-tailed test indicated that screen time had a significant association with RBMI. In addition, there was a significant association between the two techniques of screen time and physical activity measurements. The model of regression for screen time and RBMI was significant (F = 45, P = 0.000, R 2 = 0.42) and screen time explained about 16% of variance in RBMI (B = 0.021, SEB = 0.004, β = 0.325). Conclusions: Our research determined an association between screen time, physical activity and RBMI. Screen time was a prominent predictor of RBMI in children. It is important for health workers to decrease screen time of children to prevent the prevalence of being overweight and obesity.