Size distribution of submicron airborne particles can be effectively determined using electrical mobility technique. In this study, a numerical computation model for prediction of fluid flow and aerosol transport in a long column, electrical mobility spectrometer (EMS) has been developed. The internal 3D structure of an EMS [Intra and Tippayawong (2009), Korean J. Chem. Eng., 26(1), 269] was employed to simulate the complex flow patterns and aerosol particle trajectories in the EMS, including the swirling flow developed near the sheath air inlet slit. The incompressible Navier-Stokes equations were numerically calculated for the gas flow and particle trajectories, with a commercial computational fluid dynamics software package, FLUENT 6.3. The calculated results were found to agree well with previously published results in the literature. Prediction of fluid flow and aerosol transport was particularly useful in the EMS design and development