This randomized, double-blinded, controlled study began with 500 patients(144 of them removed because of incompliance). Patients those without organic bowel pathology and whom meet the Rome-III criteria, aged between 19-50 yo were enrolled in the study. Patients with known Rifaximin allergy, lactose intolerance, and those having treated for IBS whit in last 1 month as well as those with alarm symptoms (melena, fever, weight loss, anemia, rectal bleeding) were excluded. After routine biochemistry, blood count, and stool tests, Rifaximin tablet was given to a group as 1200 mg. daily in 3 divided doses, placebo tablet was given to another group in same doses for 10 days. Improvement in symptoms were scored and recorded on 10.day, 3. and 6.week. In addition to descriptive statistical methods(mean±SD), Student's-T and Mann-Whitney U-tests were used for comparison of quantitative data, chi-square and McNemar tests were used for comparison of qualitative data. Significance was accepted if p <0.05.