The purpose of this study is to test the efficacy and safety of a traditional Chinese medication Tong-Xie-Yao-Fang for patients with diarrhea-predominant irritable bowel Syndrome (IBS-D).

Participants: The sample size of this trial is 118, to test an odds ratio of 3.5 at a significance of 0.05 and power of 0.8.

Intervention: Tong-Xie-Yao-Fang versus placebo Data collection: Standard operation procedure will be developed for how to interviewing the participants, how to use the IBS diary, how to enter the data into the electronic case report forms.

Inclusion Criteria:

• Being diagnosed as IBS according to the Rome III criteria;
• Diarrhea was present for at least 75% of the time;
• With a TCM differentiation as "liver stagnation and spleen defficiency"
• Had a colonoscopy within a year to rule out other bowel diseases.

Exclusion Criteria:

• Organic gastrointestinal disease, such as colorectal cancer, inflammatory bowel disease, advanced colonic polyp;
• Had gastrointestinal surgery within a year;
• Used durgs that affect gastrointestinal motility;
• Had psychological disorder;
• Serious disease in other system, which may bias the outcome measures.