Nutritional behaviors of adolescent girls are a major health concern. Adolescents are considered main target group for improvement in nutritional behaviors. This study aimed to determine the impact of an educational intervention based on theory of planned behavior (TPB) on nutritional behaviors in Iranian adolescent girls. This randomized controlled trial study was conducted among 12-16 years old high-school girls in Tehran, Iran, from September 2015 to July 2016. The study subjects were selected using multistage random cluster sampling. Then, they were randomly assigned to the intervention (n=289) and control (n=289) groups. The school-based intervention including nutrition education program based on a modified TPB was performed among intervention group. The control group received only school routine education program. Both groups were assessed at baseline, and after 6-months follow-up. The significant improvement in attitude (15.5%, 95% CI=14.7 to 19.3), subjective norms (17.5%, 95% CI=15.1 to 0.19), perceived behavioral control (16.1%, 95% CI=14.8 to 18.5), perceived parental control (13.4 %, 95% CI=11.2 to 17.6), behavioral intention (16.4%, 95% CI=14.7 to 19.3), and behavior (17.2%, 95% CI=13.7 to 20.7) was observed in intervention group compared to control group (p<0.001 for all dimensions). This study evaluates the impact of a school-based intervention, guided by TPB, on nutritional behaviors improvement as well as knowledge of adolescent girls towards those behaviors. TPB-based educational intervention can effectively improve the healthy nutritional behaviors of adolescent girls. © by PSP.