Objective To compare the feasibility and efficacy of attempted underwater endoscopic mucosal resection（EMR） with conventional EMR for colorectal sessile or flat tumors. Methods A total of 120 colorectal tumors of 5 to 30 mm in size were removed using underwater EMR or conventional EMR. En bloc resection rates of the two groups were retrospectively analyzed. Results Underwater EMR was performed on 53 colorectal sessile／flat tumors with the median size of 22 mm， and conventional EMR was performed on 67 tumors of the same median size. The overall en bloc resection rate of the underwater EMR group was higher than that of the conventional EMR group（81.1％ VS 59.7％， P=0.012）.The subgroup analysis showed en bloc resection rates of Paris 0-Ⅱa tumor （76.7％ VS 50.0％， P=0.035） and those located at the inaccessible regions （100.0％ VS 42.9％， P=0.004） were higher than that of conventional EMR. Neither postoperative bleeding nor perforation occurred in the two groups. Residual adenoma was found in 1 patient in underwater EMR group and 2 in the conventional EMR group，respectively（P=0.712）. Conclusion Underwater EMR with prior submucosal injection appears superior to conventional EMR for the removal of colorectal sessile or flat Paris 0-Ⅰs／Ⅱa tumors of less than 3 cm in size in the rate of complete endoscopic resection and defect closure.