Number-size distribution of aerosols and its spatial variation, were observed in the free troposphere up to 11 km altitude over the northwestern Pacific Ocean, in the Pacific Atmospheric Chemistry Experiment (PACE)-7 campaign in February 2000. Characteristics of the size distributions were observed in the relation to the different air mass. The spatial difference in the features of size distributions was clearly divided by the location of subtropical front. Over the extratropical region, the mode radii were large (0.03-0.06 mm), and the concentrations of aerosols in the accumulation mode (0.15 < r < 0.5 μm) were high, suggesting a strong influence from anthropogenic particles from the Asian continent. On the other hand, the size distributions in the subtropical air mass had the mode radii of 0.01-0.03 mm, and showed low concentrations of accumulation mode particle. Moreover, high number concentrations of very fine particles (0.004-0.01 mm radius), were distributed at upper altitudes (> 8 km) over 15-31°N latitude. These aerosols were assumed to be mainly sulfuric acid particles transported from the tropics. Size distribution at the center of subtropical jet streams was slightly aged, suggesting influence from the continent. However, it is basically resemble to the size distributions measured in the upper troposphere over the subtropical region. On the basis of the trajectory analysis, these aerosols would have been transported from the tropical upper troposphere, to the middle latitudes by Hadley circulation, and then mixed into the subtropical jet streams.