Particle samples were collected in August 2004 both inside and outside Emperor Qins Terra-Cotta Museum in Xian, China. Mass and chemical composition of total suspended particles (TSP, particles with aerodynamic diameter less than ～30 μm), PM2.5 (particles with aerodynamic diameter ＜2.5 μm) were determined. The average levels Sulfate ((32.4+6.2)%), organics ((27.7+8.0)%), and geological material ((12.5+3.4)%) dominated indoor PM2.5, followed by ammonium ((8.9+2.8)%), nitrate ((7.0+2.9)%), and elemental carbon (EC, (3.9±1.5)%). Particle size distribution varied with the number of tourists in the museum. The size of sulfate, organics, EC, nitrate, and ammonium was found to vary in the range of 0.43 to 3.3 μm in fraction. Ion balance indicated that the aerosol was acidic, with insufficient ammonium ions to neutralize the sulfuric and nitric acids. High concentrations of acidic aerosols will erode the Terra-cotta warriors and horses especially in the summer season with high temperature (30℃) and relative humidity (70%) and undesirable solar radiation inside the museum. More attention should be paid to protecting these precious antiques made 2000 years ago.