Background: Endoscopic resection of benign colorectal polyps and early cancer is well established. Local staging is of paramount importance to ensure that local resection is feasible. Endoscopic ultrasound has been used to evaluate the depth of lesions in the rectum, but its use in the colon is limited., Objective: This prospective study aims to evaluate the accuracy of 20-MHz mini probe ultrasound before the endoscopic resection of colorectal tumors., Design: All patients underwent 20-MHz high-frequency mini probe ultrasound of the colorectal lesion during colonoscopic examination. The mini probes were inserted through the working channel of the colonoscope, and acoustic coupling was achieved by instilling water to completely submerge the lesion. The depth of infiltration of the colorectal tumor was identified before resection. The lesions were sent for histological examination, and the level of infiltration was compared with the preoperative ultrasound depth., Setting: This study was conducted at a tertiary referral university teaching hospital., Patients: Consecutive patients referred for consideration of endoscopic resection were included in the study., Interventions: All patients were subject to colonoscopic high-frequency mini probe ultrasound to evaluate the depth of lesion before local resection., Main Outcome Measures: There were 2 outcome measures: the ultrasound depth of colorectal lesion and the histological depth., Results: One hundred four patients were included with a mean age of 70 years. The surgical procedures included 59 endoscopic mucosal resections, 36 transanal endoscopic microsurgeries, and 9 endoscopic submucosal dissections. The 20-MHz ultrasound correctly staged 100 of 104 lesions, an overall accuracy of 96.1%. Eighty-eight of 89 mucosal lesions and 11 of 12 submucosal lesions were correctly staged., Limitations: The ultrasound examination was performed by the main author only and is therefore dependent on his experience alone., Conclusion: Colonoscopic high-frequency mini probe ultrasound has high accuracy in determining the depth of colorectal lesion and is useful before endoscopic resection.