Indoor particles and microclimate were measured in summer (August 2004) and winter (January 2005) periods inside and outside Emperor Qin's Terra-Cotta Museum in Xi’an, China. Indoor temperature ranged from 21.9°C to 32.4°C in summer and from 0°C to 5.3°C in winter. Relative humidity varied from 56% to 80% in summer and from 48% to 78% in winter. The number concentrations of particles were lower (0.3–1.0 μm) in summer, and were higher (1.0–7.0 μm) in winter. The average indoor PM2.5 and TSP concentrations were 108.4 ± 30.3 μg/m3 and 172.4 ± 46.5 μg/m3 in summer and were 242.3 ± 189.0 μg/m3 and 312.5 ± 112.8 μg/m3 in winter, respectively. Sulfate, organic matter, and geological material dominated indoor PM2.5, followed by ammonium, nitrate, and elemental carbon. Several milligram of sulfate particles can deposited in the museum per square meter each year based on the dry deposition estimate. High concentrations of acidic particles suspended inside the museum and their depositions have high risk for the erosion of the terra-cotta figures.