Particle inertial deposition in a tube with an abrupt contraction was studied experimentally and numerically at low pressure. Measurements were performed for particle deposition onto an orifice plate (orifice diameter: 1.16cm) placed in a tube (inner diameter: 3.48cm). The system pressure ranged from 27 to 37Pa (0.2-0.28Torr) and the flow Reynolds number, Re, in the tube was 3.0. Spatially uniform aerosols were produced in the low-pressure environment by the method developed by Sato et al. (Aerosol Sci. Technol., 2001). The observed deposition-efficiency curve as a function of the Stokes number, Stk, is different from that obtained for Re≥100 at atmospheric pressure (J. Aerosol Sci. 21 (1990) 29; J. Aerosol Sci. 26 (1995) 563). The difference was confirmed by numerical studies. The numerical results for Re=0.1-30 revealed that the deposition curve shifts to a larger Stokes number as the Reynolds number decreases due to flow-pattern changes near the orifice plate. An empirical deposition-efficiency curve was obtained as a function of the Stokes number, the Reynolds number, and the contraction ratio (the ratio of the tube inner diameter to the orifice diameter), R.