Cloud condensation nuclei (CCN5) concentrations and critical diameters (Dc5) at 0.5% supersaturation were continuously measured and corresponding hygroscopicity parameter (κ5) were calculated at Tsukuba during between January and March, 2016. The κ5 values were representative of particles in Aitken size ranges (approximately 50–100 nm diameter). In the periods just after the occurrence of new particle formation events, the κ5 values were widely ranged (0.01–0.6), however, they tended to concentrate into a narrow range of 0.05–0.15 as the particles grew larger. This tendency was explained partly with increasing mixing ratios of HOC and Char-EC, and decrease of SO4 in PM2.5. Internal mixing of many organic compounds and oxidation of SOA were also possibly effective to the concentrating tendency of κ values. According to examinations using electron microscope, extraordinary high values of κ were caused by high concentrations of ammonium sulfate concentrations (κ～0.4) or influences of biomass burnings (κ～0.2).