Indoor particulate matter exposure is one of the main environmental risk factors influencing human health. Previous studies have indicated that the degree of hazards about ultrafine particles was associated with the particle surface area and number concentration. Polydisperse submicron particles (PSPs) contribute most of the indoor aerosol particle number and are usually used for removal testing. However, the behaviors of monodisperse submicron particles (MSPs) are easier to analyze. This study aims to investigate the difference between the deposition removal efficiency of MSP and PSP by the negative air ionizer (NAI) under different settings of freestream air velocity (FAV). The particle-removal experiments were conducted in a stainless steel test chamber under the condition of 50% relative humidity and FAV of 0.56, 1.2 and 2.0 m/s. The NaCl and oleic acid PSPs were generated from a constant output atomizer and the NaCl MSPs (30, 50, 100, 170 and 300 nm) were generated by passing the polydisperse NaCl aerosol through a Differential Mobility Analyzer. The concentration and size distribution of the particles in the chamber were monitored with a Scanning Mobility Particle Sizer (SMPS). The decay constant of particles number concentration, particle removal efficiency (PRE), enhancement factor of deposition rate (EF) and air cleaner effectiveness (ACE) were determined from the SMPS data. The decay constant of particle number concentration increased with the decrease of FAV and decreased with the increase of particle size. Both of the PREs of MSP and PSP were quite high, especially for the smaller particles and under low FAV. The EFs ranged from 4.4 to 115.5 and were proportional to the particle size. In addition, the ACE ranged from 0.57 to 0.96 and increased as the particle size increased. These results give a better understanding of the deposition of 30– 300-nm particles enhanced by the NAI and the influential factors.