The size-fractionated particle number concentrations were monitored continuously in the different exhibition halls of an enclosed museum, in Shanghai. Particles in different size bins were collected to investigate the main chemical compositions. Results indicated that the mass concentrations of coarse particles, fine particles and ultrafine particles were in the range of 20.50–24.38 μg/m3, 23.39–24.08 μg/m3 and 16.02–17.48 μg/m3, respectively. In addition, the activities of cleaning and walking can greatly elevate the number concentration of coarse particles by a factor of about 8–172 times. The decay rates of size-fractionated particle increased with the enlarging particle diameter, ranging from 2 × 10–5 to 8 × 10–4 s–1. The mass concentration of SO42–, NO3–, NH4+ peaked within the particle size range of 0.32–0.56 μm. The ratios of [NO3–]/[SO42–] on the National Day and working day were 0.08 and 0.12, respectively. Comparatively, the peak concentrations of Na+, Cl–, K+, Mg2+ and Ca2+ occurred in the various size ranges. Total organic acids distributed uniformly, the acetate ion mass concentration was 1.238 μg/m3, which was higher than both formate ion and oxalate ion concentrations. The concentrations of OC and EC showed similar trends on both the National Day and working day, and greater OC/EC ratios were found in the particles within the size range less than 0.1 μm and larger than 1 μm.