Introduction: Constipation is highly prevalent, often chronic gastrointestinal disorder that affects adults. The treatment with classic drugs did not cut, in one hand with the inadequate relief of bloating and other symptoms, and with the luck of efficacy in relieving constipation. Therefore, the search for novel safe laxative drugs seems, inevitable. Rheum undulatum L. was traditionally used in constipation, thus we have attempted to evaluate the laxative effect of Rheum undulatum L. Purpose: The laxative effect of Rheum undulatum L. was evaluated against loperamide induced constipated rats. Methodology: Fifteen male normal rats were used in this study. Fifteen male constipated wistar albino rats weighing 180-250 g were also used for the study and randomized into three groups (n=5) in each of the experiments. Constipated control group rats oral administrated distilled water. Constipated rats (treatment groups) were treated with 4.1 mg/kg dose body weight /day of the preparation for one day and also Laxing a standard drug was used for the reference group. The fecal weight, the fecal humidity laxative activity were monitored in experimental rats.Results: Constipation was successfully induced in the rats by loperamide as seen in the elevated fecal properties compared to the control rats. The Rheum undulatum L. compounds preparation administered orally produced significant laxative activity and reduced loperamide induced constipation in dose dependent manner as seen in the increase of fecal output. The same doses of the Rheum undulatum L. compounds preparation produced a significant increase (P<0.05) fecal weight, the faeces humidity. The effect of the compounds preparation compares favourably well with Laxing, a standard laxative drug. Conclusion: The results of this study justify the use of Rheum undulatum L. compounds preparation as a laxative in traditional medicine. The produced significantly increase in fecal output of rats and the stimulation of gastrointestinal motility. Keywords: Laxative, gastro intestinal motility, loperamide, constipated