Background: The aim of this study was to determine the efficacy of two methods of colon preparation for colon cleansing in a randomized controlled trial. Methods: In this prospective randomized investigator-blinded trial, consecutive outpatients indicated for elective colonoscopy were randomized into two groups. Patients in Senna group took 24 tablets of 11 mg Senna in two divided doses 24 hour before colonoscopy. In Poly Ethylene Glycol (PEG) group they solved 4 sachets in 4 liters of water the day before the procedure and were asked to drink 250 ml every 15 minutes. The overall quality of colon cleansing was evaluated using the Aronchick scoring scale. Difficulty of the procedure, patients′ tolerance and compliance and adverse events were also evaluated. Results: 322 patients were enrolled in the study. There was no significant difference in the quality of colon cleansing, patients′ tolerance, compliance and the difficulty of the procedure between two groups (p > 0.05). The incidence of adverse effects was similar between two groups except for abdominal pain that was more severe in Senna group (p < 0.05) and nausea and vomiting that was more common in PEG group (p < 0.05) Conclusions: In conclusion we deduce that Senna has the same efficacy and patient′s acceptance as Polyethylene glycol-electrolyte solution (PEG-ES) and it could be prescribed as an alternative method for bowel preparation.