PURPOSE: Disease in small airways <2 mm in diameter is a major pathology of chronic obstructive pulmonary disease (COPD). However, compared to airways <1 mm in diameter, the pathophysiological role of airways 1-2 mm in diameter remains unclear. This study analysed phantom and human COPD data to test the hypothesis that ultra-high-resolution computed tomography (U-HRCT) can accurately measure peripheral airways that are difficult to measure with conventional CT. METHOD: The lower limit of lumen sizes measurable on U-HRCT was determined using phantom tubes. In the cross-sectional data of 110 males with COPD who underwent U-HRCT (1024 × 1024 matrix, 0.25 mm slice thickness) and spirometry, all 3rd (segmental) to 6th generation airways of the right apical and basal posterior bronchus (RB1 and RB10) were analysed. RESULTS: The errors in measuring the lumen area (LA) of phantom tubes ≥1.3 and 1.0 mm in diameter were within ±10 and -24%, respectively. The internal diameters for 70 and 62% of the 6th generation RB1 and RB10 airways were <2 mm. The numbers of 6th generation RB1 and RB10 airways decreased as the airflow limitation severity increased. Among the mean LA and sum of LA(sum-LA) of the 3rd to 6th generation airways, the sum-LA of the 6th generation had the largest impact on airflow limitation. CONCLUSIONS: U-HRCT enables accurate and direct evaluation of peripheral airways 1-2 mm in diameter. The 6th generation airways are commonly <2 mm in diameter, and the sum-LA can be a useful CT biomarker that reflects airflow limitation in COPD.