A mixing-type condensation nucleus counter (MTCNC) was used to detect nanometer-sized particles suspended under low-pressure conditions. The experimental results show that MTCNC is capable of measuring nanoparticles smaller than 10 nm at low pressures above 65 Torr (8.644 kPa) by adjusting the rate of the aerosol flow, the amount of vapor and temperature depending on the desired operation pressure. At pressures above 160 Torr (21.33 kPa), the optimal operation conditions of a MTCNC, variables such as the saturator temperature, condenser temperature and mixing ratio are similar to those used at atmospheric pressure. Since the condensational growth of particles depends greatly upon the residence time, the current MTCNC needs improvement to increase the growth time under even lower pressure conditions. At the optimal operation conditions, the counting efficiencies for nanoparticle size 3 nm particles are 85% at atmospheric pressure and 40% at an operating pressure of 115 Torr (15.33 kPa), respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.