To date, few comprehensive field observations of new particle formation (NPF) have been carried out at moun-taintop sites in China. In this study, simultaneous measurements of particle size distribution, trace gases, meteorological parameters, and mass concentration and chemical composition of PM 2.5 were performed at the summit of Mt. Tai (1534 m a.s.l.) from 25 (Phase III) to investigate characteristics and favorable conditions of NPF in a relatively clean mountain-top environment. The NPF events were identified based on particle size distribution measured by the neutral cluster and air ion spectrometer (NAIS), and 66 such events were observed during a period of 164 days-corresponding to an occurrence frequency of 40 %. The formation rates of 3 nm particles (J 3) and growth rates were in the ranges of 0.82-25.04 cm −3 s −1 and 0.58-7.76 nm h −1 , respectively. On average , the condensation sink (CS), O 3 concentration, air temperature , and relative humidity were lower, whereas the SO 2 concentration was higher on NPF days than that on non-NPF days. The CS on Mt. Tai was at a low level and lower CS was critical for NPF. NPF events were common when wind came from the east-southeast and west-southwest, which was probably associated with relatively lower CS in the east-southeast and higher SO 2 concentration in the west-southwest. O 3 was not a governing factor for NPF in this study, and a high level of NO x concentration might be responsible for the decreased O 3 concentration on NPF days. Three categories of backward trajectories were classified, among which the continental air mass was the majority. The continental air mass passing through more polluted areas (denoted as Type I) favored NPF because of enhanced SO 2 concentration and potential ammonia with it. An in-depth analysis of SO 2 indicated that sulfuric acid was a dominant precursor on Mt. Tai; meanwhile, biogenic organics released from ambient forests in warm seasons and anthropogenic volatile organic compounds emitted from domestic heating in cold seasons also promoted NPF.