Background & Aims: Asthma is a chronic inflammatory disease of the airways which is associated with the triad of cough, wheezing and dyspnea. Allergic asthma is often associated with a personal or family history of allergic diseases and mostly begins at early childhood. Mold is one of the most common irritants of asthma and allergic rhinitis. In the last 20 years there has been 30% increase in the prevalence of moldsrelated asthma. Methods: In this study the prevalence of mold allergy in asthmatic patients of less than 18 years old who refrred to Semnan allergy clinic during 2007-2008 was determined. Skin prick test with standard mold allergens was performed and sensitivity to the most prevalent molds like Alternaria, Penicillium, Aspergillus, cladosporium and mucor was evaluated. Results: From 220 studied subjects, 35% (95% Confidence Interval: 28.7-41.3%) were shown to be sensitive to at least one mold. The prevalence rates of sensitivity to molds were as follow: Alternaria14.5%, Aspergillus 12.7%, Cladosporium 13.2%, Penicilium 12.7% and Mucor 2.7%. There was a significant relationship between sensitivity to molds and the grade of asthma (P<0.05) and episodes of admissions (P<0.05), but mold sensitivity showed no significant relationship with age, gender, personal or family history of other allergic diseases and the duration of asthma. Association between grade of asthma and episodes of admissions with the number of sensitizing molds was significant (P<0.05). Conclussion: Since asthma is the most common childhood disease and molds especially Alternaria and Cladpsporium are among the most important triggers of asthma and its progression, detection of this type of allergens and warning patients to avoid them could be an effeicient way to prevent and control the disease progress.