Objective To evaluate the clinical value of acetic acid with narrow-band imaging ( NBI ) and magnifying endoscopy ( ME ) on diagnosis of small colorectal polyps. Methods In this prospective study, 261 small colorectal polyps from 122 patients were observed by ME, NBI-ME, and acetic acid with NBI-ME, and then received endoscopic treatment. Endoscopic images were stored electronically and randomly allocated to 3 experts and 3 non-experts for diagnosis using Kudo pit pattern. The postoperative pathologic results acted as gold standard to evaluate the diagnostic accuracy of different endoscopic modes for small colorectal polyps. The image definition and interobserver agreement were compared among different endoscopic modes. Results The diagnostic accuracy of ME, NBI-ME, and acetic acid with NBI-ME for small colorectal polyps was 65. 5％ ( 171/261) , 90. 0％ ( 235/261) , and 94. 6％ ( 247/261) , respectively, in the experts group, and 57. 1％ ( 149/261) , 83. 1％ ( 217/261) , and 89. 3％ ( 233/261) , respectively, in the non-experts group. All experts and non-experts diagnosed small colorectal polyps more accurately by acetic acid with NBI-ME than by NBI-ME ( all P<0. 05 ) and ME ( all P<0. 001 ) . The image definition scores of acetic acid with NBI-ME in the experts group and non-experts group were significantly higher than those of NBI-ME and ME ( all P<0. 001) . The results of interobserver agreement showed that Kappa values (95％CI) of ME, NBI-ME, and acetic acid with NBI-ME diagnosis were 0. 578 (0. 508-0. 648), 0. 669 (0. 599-0. 739), and 0. 940 (0. 870-1. 010), respectively, for experts and 0. 476 (0. 406-0. 546), 0. 534 ( 0. 464-0. 604) , and 0. 830 ( 0. 760-0. 900 ) , respectively, for non-experts. Acetic acid with NBI-ME showed good interobserver agreement. Conclusion Acetic acid with NBI-ME has a higher diagnostic accuracy and good reproducibility for colorectal small polyps compared with ME and NBI-ME.