Background: Colonoscopy has become essential procedure for detection and treatment of colonic lesion. Good bowel preparation is essential for adequate colonoscopy. Oral sodium phosphate (NaP) is a laxative that has demonstrated efficacy and safety for bowel preparation. However, sodium phosphate produces dehydration.Objective: To compare the quality of bowel preparation, its side effects and patient acceptance of the two bowel preparation regimens. Method: Patients undergoing elective colonoscopy were sequentially randomized to receive either two dose NaP, or bisacodyl addition single dose NaP before colonoscopy. The endoscopists who record the qualityof bowel preparation were blind to preparation regimens. The questionnaire was completed by patients to assess tolerance, acceptability and side effect of the preparation.Result: A total 151 subjects were enrolled. Overall, bowel cleaning rated “excellent” or “good” in 89.2% of two dose NaP group compare with 84.5% of single dose with bisacodyl. There were no significant difference in quality of preparation (p=0.068). Subject in two dose group demonstrated significantly less residual stool (p<0.05). Nausea and weakness were significantly more common in two dose NaP group.Conclusion: One dose NaP with bisacodyl is well tolerated, safe and provide bowel cleansing similar to two dose regimen but visibility of lumen can be impaired because more residual stool , but patient rated one dose with bisacodyl preparation well acceptance because reduced amount of NaP solution and taking the small tablet is easier.Bisacodyl addition one dose NaP should become an alternation preparation for patientundergoing elective colonoscopy.