A modified version of the previously developed tubular particle-removal device based on the UV:photoelectron method (Shimada et al. (1997) J. Aerosol Sci. 28, 649) is manufactured to investigate removal efficiency of aerosol particles suspended in air at a reduced pressure down to 20 torr. The present measurements with monodisperse particles of 0.458 mu m in diameter show the dependence of the removal efficiency on pressure, Bow rate and electric field in the electrostatic precipitation device. To predict the removal efficiency, a theoretical analysis for the particle collection processes in the precipitator is made, in which particle charging by negative ions, and particle transport by air Bow, diffusion and electrical migration are numerically calculated taking into account the effects of reduced pressure. The calculated results reproduce the measured removal efficiencies well. It is found in the calculation that the Fuchs＇ approach to ionic particle charging needs modification to predict charging of highly charged particles by ions of the same polarity. It is also demonstrated that the removal efficiency is increased to 100% when the precipitator is equipped with an additional set of electrodes that collects charged particles.