Effect of Brownian diffusion on the electrical classification of ultrafine particles in the differential mobility analyzer (DMA) has been studied theoretically and experimentally. Two kinds of particle losses which are undesirable in size analysis, (1) loss caused by the Brownian diffusion of particles traversing the sheath air stream toward the collector rod of the DMA, and (2) loss caused by Brownian diffusive deposition of particles on every wall of the DMA excluding the collector surface, were theoretically evaluated by solving the diffusion equation. Some of the calculation results were confirmed by experiments for particles of various sizes, two different lengths of DMA and various ratios of flow rate of aerosol to sheath air.