Introduction: Until now, numerous effective compounds of black seed (Nigella sativa) therapeutic benefits and its active ingredient thymoquinone are known; However, effect of this drug to treat and improve clinical symptoms of patients with functional bowel disorders, including IBS, has not been investigated. Present study was conducted with the aim of investigating effectiveness of black seed extract in patients with IBS treatment. Material and Method: This randomized controlled clinical trial study was conducted on 62 patients with IBS in 1400. The patients were randomly divided into two groups receiving capsules containing 100 mg of black seed extract (test group; 32 people) or similar capsules containing placebo (control group; 30 people) for a period of 8 weeks. The symptoms severity was evaluated based on IBS severity questionnaire (including abdominal pain severity, abdominal distension severity, degree of satisfaction with bowel habits, impact of IBS on quality of life). The data were analyzed with the help of statistical software