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Background and Objective: Children constipation is one of the common causes of abdominal pain. Cow´s milk in children diet is suggested to be one of the main cause of chronic constipation. This study was performed to determine the effect of cow´s milk elimination from children diet with chronic constipation. Methods: This quasi-experimental study, was performed on 80 children with chronic constipation. Rome-III criteria were considered for disease diagnosis. Cow’s milk was eliminated for three weeks from children’s diet and the clinical symptoms in children were surveyed, subsequently. Results: Constipation was cured in 33.8% of children as result of cow's milk free diet. The rate of treatment response in affecting children whom their disease in began under 2 years of age (52.6%) was significantly higher than others (16.6%) (P<0.05). Children treatment was not significantly related to child’s gender and amount of milk consumption. Conclusion: Eliminating of cow's milk from the diet improves the chronic constipation in children.