We assessed the effect of multi-species probiotic mixture on the changes in fecal microbiota and irritable bowel syndrome (IBS) symptoms. Eighty-one IBS patients were randomly assigned to receive either probiotic mixture (n = 39; containing Lactobacillus acidophilus, L. rhamnosus, Bifidobacterium breve, B. actis, B. longum, and Streptococcus thermophilus) or placebo (n = 42) for 4 weeks. A questionnaire regarding general symptom relief was administered. The change in total symptom scores (sum of 10 IBS symptoms) and subtotal scores in 4 domains (pain, constipation, diarrhea, and bloating/gas) were evaluated. The change in fecal flora was determined by quantitative real-time PCR. The concentration of probiotic strains significantly increased after ingestion in probiotics group (B. bifidum, p = 0.043; B. lactis, p<0.001; L. acidophilus, p = 0.016; L. rhamnosus, p<0.001). The proportion of patients with adequate symptom relief was higher in probiotics group than in placebo group (74.4% vs 61.9%, p = 0.230). The decrease in total symptom score over time was not significantly different between the groups (p = 0.703). Among subtotal scores of 4 IBS symptom domains, the time effect was significantly different for diarrhea-symptom score between the groups (p = 0.017). A 4-week administration of multi-species probiotic mixture significantly increased the fecal concentration of most probiotic strains and improved diarrhea-symptom scores in IBS patients.