Background The purpose of this study was to elucidate the effects of a dual-coated probiotic supplement (Duolac Care) on symptoms of diarrhea-predominant irritable bowel syndrome in a randomized double-blind clinical trial. Methods Fifty subjects with diarrhea-predominant irritable bowel syndrome were randomly assigned to either the non-coating group or the dual- coating group in order to receive two capsules per day of multi-species probiotics containing 5 billion bacteria per capsule for 4 weeks. Data from an adequate relief questionnaire were used in assessment of primary outcome. Daily records of stool frequencies and the Bristol stool scale, a weekly symptom diary using 100-mm visual analog scale, and Beck depression inventories were collected. Blood tests including blood cell counts, interleukin-10, tumor necrosis factoralpha and inducible nitric oxide synthase, and regulatory T cells-CD4 + CD25 (high) + T cells, CD4 + LAP + T cells and CD25 (high) + LAP + T cells-were analyzed before and after the study. The shift of gut microbiota was investigated using a quantitative real-time polymerase chain reaction assay. Results Responses to the adequate relief questionnaire indicated significant improvement in overall discomfort in the dual-coating group and the ratio of normal stools to hard or watery stools had a better effect from dual-coated probiotics compared to non-coated probiotics. This may be due to a shift of intestinal microbiota, as our correlation analysis showed significant negative correlation between Bifidobacterium and urgency of defecation. Conclusions Our result implies that dual-coating layers of probiotic supplement can be a candidate for treatment of diarrhea-predominant irritable bowel syndrome.