Introduction: Asthma is the most common chronic disorder in children and is more prevalent in children 5-10 years of age. The prevalence of childhood asthma has increased in recent years. Food allergy has been implicated as a risk factor for asthma. In this study, we determined the prevalence of food allergy and associated factors in asthmatic children less than 18 years of age.Materials and Methods: In this Cross-sectional study, we studied 298 asthmatic children less than 18 years of age in Semnan (Iran) in 2007-2008, who were diagnosed according to physical examination and history. Skin prick test was performed for all of participants and a questionnaire was filled out. Wheals of 3 millimeter and flares of 10 millimeters larger than negative control was regarded as a positive test.Results: 35.9% (95% Confidence Interval: 30.5-41.3%) of asthmatic children showed sensitization to at least one of the principal allergenic foods (wheat, rice, peanut, egg, soya and cow milk). Prevalence of food allergy in asthmatic children under 18 years of age had significant association with gender, birth weight, history of other allergies, family history of allergy, type of coexistent allergy and age of initiation of symptoms, age of introduction of complementary feeding, consumption of cow milk before one year of age and also duration of breast feeding( P<0.05).Conclusion: these finding showed that the prevalence of food allergy in asthmatic children is high. There is significant association between food allergy and use of cow's milk before one year of age, low birth weight, short breast feeding period, male gender, history of other allergies in the patient and close relatives, early age of initiation of symptoms and also introduction of complementary foods before six month of age. Thus, we should have particular attention on food allergens in such kind of patients, until they outgrown their food allergies. This is the optimum way for controlling asthma in these people.