Irritable bowel syndrome (IBS) is one of the most common functional gastrointestinal disorder which is associated with considerable sufferings of patient and Peppermint oil is volatile oil, its active principle is menthol-contain a cyclic monoterpine which has anti-spasmotic properties due to its ability to block calcium channel of intestinal smooth muscles. This study observed the efficacy of peppermint oil for relieving the symptoms and changes of quality of life (QOL) in diarrhea predominant IBS. This was a prospective double blind randomized placebo-controlled study conducted in the Bangabandhu Sheikh Mujib Medical University during July 2008 to September 2009. Patients who fulfilled ROME II were initially selected but those had red flag signs or any organic disease was excluded from the study. Seventy four patients were enrolled in the study and randomly allocated to receive either peppermint oil or placebo three times daily for six weeks. Changes of symptoms were assessed three week interval during treatment and two weeks after the end of treatment. Data were analyzed by paired and unpaired 't' test. Finally sixty five patients completed the trial. It was observed that, at six weeks of therapy abdominal pain is markedly improved (mean±SD) 4.94±1.30 in peppermint oil group compared with 6.15±1.24 in placebo group and the difference was statistically highly significant (p>0.001). But two weeks after end of trials pain score again increased (6.09±1.93). Other symptoms and quality of life did not improve significantly. So the study result concludes that peppermint oil is effective in reliving only abdominal pain in diarrhea predominant IBS transiently.