Introduction: While missing breakfast has been known to result in declines in cognitive function, not all studies show positive effects of breakfast consumption on cognitive function. Differences in breakfast composition may account for some of the contradictory results across studies. However, since we could not find any published study from Iran investigating the effect of breakfast composition on cognitive performance, this study designed to assess the effect of 2 common Iranian breakfast foods (high carbohydrate, high protein) on the cognitive performance of elementary school children. Material and Methods: Fifty-one well-nourished children (27 girls) aged 9-11 years took part in the present study. Children consumed one of 2 breakfasts (subjects) or took no breakfast (controls) in a randomized cross-over design with one week in between for a wash-out period. The two breakfasts were high carbohydrate (60%) and high protein (25%), similar in energy and fat, but differing in carbohydrate and protein composition. Cognitive performance was assessed prior to and 30 and 120 minutes after breakfasts, using a selection of tests from the CDR standard computerized assessment system. Statistical analysis was conducted by using SPSS (version 15) and P<0.05 was considered as the level of significance. Results: Results indicated that all 3 treatments (high carbohydrate, high protein, fasted state) were followed by decline in cognitive performance in both sexes, with no significant differences between the 2 groups, in most of the tasks. Conclusion: Our results did not show the effect of breakfast composition on the memories of well-nourished children and further research should address the extent to which different types of breakfast influence different domains of cognitive function. Also quality and quantity of breakfast should be assessed besides cognition, because of their effect on mood.