Abstract: Theory and design of a new electrical-mobility based instrument for measurement of aerosol particle size distributions in real time is presented. Miniature Electrical Aerosol Spectrometer has a rectangular cross-section with two main regions: the Electrostatic Precipitator (ESP) and Classifier sections. The ESP section enables charged particle injection into the classifier section in a narrow range of streamlines at the desired location. The injected charged particles are then segregated based on their electrical mobility in the classifier section and collected on a series of plates that are connected to electrometers. Real-time particle size distribution measurements can be inferred from the electrometer signal strengths with the knowledge of the instrument transfer function.