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Background and Objective: breakfast is recommended as the most important meal of the day, but not all studies show positive effects of breakfast consumption on the cognitive performance. Differences in breakfast composition may account for some of the contradictory results reported so far. There are limited studies investigating the effect of breakfast composition on cognitive performance. This study was designed to compare the effect of two common Iranian breakfasts (high-carbohydrate vs high-protein) on the cognitive performance of elementary school children Materials and Methods: Fifty-one well-nourished children (27 girls, 24 boys) aged 9-11 years participated in the this study. They consumed one of 2 breakfasts (60%carbohydrate or 25%- protein), or took no breakfast, as controls (the fasted state) in a randomized cross-over design with one week apart as the wash-out period. The breakfasts were similar with regard to the energy and fat contents, but differed in carbohydrate and protein composition. Cognitive performance was assessed prior to, and 30 and 120 minutes after, taking breakfast, using the standard computerized CDR assessment tests. Statistical analysis was conducted using SPSS (version 15), and P<0.05 was considered as the level of significance. Results: No statistically significant differences were observed among the 3 groups (high-carbohydrate breakfast, high-protein breakfast, and control) with regard to most of the tasks. However, the high-carbohydrate breakfast significantly impaired the simple reaction time (P<0.023), the choice reaction time (P<0.018), and the power of attention (P<0.013) in girls. The choice reaction time improved in the control boys (P<0.003). Conclusion: Breakfast composition had no effect on the healthy boys' cognitive performance, but a high-carbohydrate breakfast reduced the power of attention in healthy girls. It is recommend that the effect of quality and quantity of breakfast on the mood of the children be also assessed in future studies.