Particle number size distribution (PNSD) is an important variable for evaluating the effect of aerosols on climate. In this study, the PNSD in the size range of 3 nm–2.5 µm was measured over a 20-month period at Mt. Tai station, located at ~1500 m asl. in central east China (CEC). The mean particle number concentrations in the nucleation (Nnuc, 3–25 nm), Aitken (NAit, 25–100 nm), accumulation modes (Nacc, 100–1000 nm) and in total measured particle size range were 3200 cm–3, 5200 cm–3, 3400 cm–3, and 11800 cm–3, respectively. New particle formation (NPF) events determined from the PNSD data occurred on 32% of measured days, with particle formation rate and growth rate of 4.0 ± 3.7 cm–3 s–1 and 6.1 ± 2.5 nm h–1, respectively. Time periods of 12:00–17:00 and 23:00–7:00 local time were selected to represent periods when the air mass at the station was dominated by planetary boundary layer (PBL) and free troposphere (FT) air, respectively. The diurnal variation of particle number concentration was influenced mostly by NPF events and evolution of the PBL. When NPF event days were excluded, the particle number concentration also experienced a seasonal variation with maximum in summer and minimum in winter. This seasonality was influenced by seasonal variations in PBL evolution and by air mass advection. The results of this study characterize the regional particle climatology in central east China in terms of particle number and size.