We evaluated the usefulness of the high-frequency ultrasound probe (HFUP, 20 MHz) to determine the depth of tumor invasion in 45 patients with superficial colorectal tumors. The correct diagnostic rate was 66% (30/45) when the depth of tumor invasion was classified into the following 6 layers: mucosa (m), upper 1/3 (sm1), middle 1/3 (sm2), and lower 1/3 (sm3) areas of the submucosa, muscularis propria (mp), and the subserosa or deeper areas (s). However, when the depth of tumor invasion was evaluated in 3 layers (m-sm1, sm2-sm3, and mp-deeper layer), which is the classification used to select cases for endoscopic mucosal resection, the correct diagnostic rate was 88.9% (40/45). These results suggest that the HFUP is useful to determine the depth of invasion to select treatment for superficial colorectal tumors.