A system for measuring nm-sized aerosol particles under low pressure conditions using a differential mobility analyzer (low pressure DMA, LPDMA) with a Faraday cup electrometer (FCE) as the detector is developed. The performance of the LPDMA is investigated experimentally using a tandem DMA technique. It is shown that LPDMA can be used to measure nm-sized particles (4-10 nm) under low-pressure conditions (60-760 Torr). The effect of Brownian diffusion on the performance of LPDMA is qualitatively discussed by comparing the results obtained using LPDMA with those of conventional and Vienna DMAs with dimensionless parameters. The equilibrium charge distribution of aerosol particles reached under low-pressure conditions is also evaluated by solving population balance equations for particle-ion collision. The LPDMA/FCE system is also applied to the measurement of nm-sized silver particles produced by physical Vapor deposition (PVD) and TiO2 particles produced by low pressure chemical vapor deposition (LPCVD).