Background: Dietary patterns are influenced by behavioral factors in early adulthood. Nutritional education has a major role in improving health status of adolescents. Objective (s):This study aimed at assessing the knowledge, attitude and behavior about healthy dietary habits among adolescent females before and after a nutrition education program in Riyadh, KSA. Methods: An intervention study was conducted in two schools among 105 students (57 from intermediate school and 48 from high school). Nutrition-related knowledge, attitude and behavior were assessed pre-intervention and one week post-intervention using the same instrument. The nutrition education program consisted of three sessions This study was approved by Institutional Review Board (IRB) of Princess Nourah bint Abdulrahman University. Results: After the intervention, there was significant improvement in students' attitude (from 21.6±5.4 to 23.7 ± 3.2) and behavior towards healthy dietary habits (from 48±12.1 to 51.5 ± 7.2) with a P-value of 0.02 and 0.006 respectively. Knowledge score did not improve after the intervention. There was a significant decrease in the total barrier score from 12.2 ± 2.7 to 11.2 ± 2.7 with a pvalue of 0.01, and significant change in the consumption of daily breakfast, fresh food and whole grain (p-value 0.01, 0.05 & 0.02 respectively) and in checking expiry date of food (p-value 0.03) The Avoidance of fatty meals also improved significantly (p-value 0.03) . Conclusion: This nutrition education program was effective in improving adolescents' attitude and behavior in relation to healthy diet.