Aerosol penetration efficiency is one of the most important parameters for the safety assessment of a containment shell or containers of radioactive materials. In this work, a practical procedure has been developed to study the penetration efficiency of fine aerosol particles (size < 300 nm) through capillaries with bore sizes ranging from 5 to 20 pm and lengths from 10 mm to 80 mm under pressure differences from 60 to 450 kPa. The effects of the upstream aerosol concentration, capillary dimension, and pressure difference on the aerosol penetration were investigated, and the relationships between the aerosol penetration efficiency and the air leakage rate as well as the average flow velocity in the capillary were obtained. The results showed that the penetration efficiency of aerosols through a capillary was positively related to the pressure difference as well as the air leakage rate until reaching 100%. For a given air leakage rate, the aerosol penetration efficiency fluctuated within a relatively wide range due to the influence of the pressure difference and capillary dimensions. The aerosol penetration efficiency decreased significantly with increasing capillary length but was identical for capillaries of the same length but different bore sizes. The aerosol penetration efficiency in capillaries showed a better correlation with the average flow velocity than with the air leakage rate. For flow velocities below 5 m/s, the aerosol penetration efficiency increased with the air velocity in the capillary, while for velocities above 5 m/s, a constant level of approximately 80-100% was maintained.