OBJECTIVE: To evaluate the use of a small mobile ultrasound probe to localize small lung tumors during thoracoscopic surgery under thoracic CO2 insufflation. METHODS: We prospectively enrolled 20 patients (26 tumors) scheduled to undergo thoracoscopic pulmonary wedge resection between April 2016 and October 2018. Ultrasonographic tumor detection was performed with an ARIETTA 850 and L51K probe (Hitachi, Tokyo, Japan). Ultrasonography was repeated after achieving adequate lung collapse under a positive intrathoracic pressure of 8-15 mmHg. The appearance on preoperative CT versus the ultrasonographic localization was compared for each tumor. The receiver operating characteristic curves were compared for the tumor dimension of the lung window, consolidation dimension of the lung window, tumor dimension of the mediastinal window (MD), and tumor depth from the lung surface. RESULTS: The average age was 62 years (range 42-79 years), average pathological tumor size was 9 mm (range 3-22 mm), and average tumor depth was 6 mm (range 1-25 mm). Although no tumors could be visualized before lung collapse, 22 tumors (85%) were detectable with ultrasonography after lung collapse. Of these 22 tumors, 16 were well-depicted, while six were poorly delineated. MD showed the largest area under the receiver operating characteristic curve (0.81), and tumors with a MD of ≤ 6 mm had a high risk of difficult localization using ultrasonography. CONCLUSION: This ultrasonographic method obtained high tumor detection rates, especially for tumors with a MD > 6 mm. Tumors with a MD ≤ 6 mm may require another localization method. CLINICAL REGISTRATION NUMBER: University Hospital Medical Information Network Clinical Trials Registry (UMIN000036921).