A small mixing-type unipolar charger for nanoparticles (SMUC) was developed to increase the charging efficiency and throughput by using a small charging chamber (0.5 cm3) to reduce particle loss caused by electrostatic dispersion. The two main components of the SMUC are a high-pressure corona ionizer (HPC Ionizer) in which high concentration unipolar ions are generated, and a charging chamber in which the unipolar ions are mixed with nanoparticles without an external electric field. The charging performance of the SMUC was evaluated experimentally by measuring the charging ratio fc and the penetration ratio fp. For positive charging in the particle size range of 5-40 nm, fc+= 0.91-0.95 and fp += 0.71-0.90, at an ion concentration of Ng +-5.25 × 109 ions/cm3 and a charging time tR = 0.025 s. For negative charging, fc-= 0.74-0.93 and fp-= 0.73-0.85, at Ng-= 6.24 × 109 ions/cm3and tR = 0.025 s. Based on these results, the extrinsic charging efficiency defined as fe = fc×fp /fd (where fd is the dilution rate) was fe-= 0.46-0.66 and fe += 0.59-0.71 for negative and positive charging. The f c+ and fc- were estimated theoretically with and without electrostatic dispersion to clarify the effect of electrostatic dispersion on particle losses in the charging chamber. The measured fe for dp< 10 nm in the 0.5 cm3 chamber is higher than the theoretical results without electrostatic dispersion, indicating that particle loss caused by electrostatic dispersion is reduced by use of the small volume charging chamber.