A miniature optical sensor, the core of a miniature laser aerosol particle counter, has been developed. The optical system of the sensor is a right-angle scattering optics. A high-power laser diode module is used as the light source, and a PIN photodiode with high performance is used as the photo-detector. By using a reflective spherical mirror with large numerical aperture, the light scattered by individual particles is collected over a large angular range of 20° to 160°. The scattered light signal from the particle is short pulse, which consists mainly of high frequency components. A band-pass preamplifier is used to eliminate lower-frequency eletromagnetic interference from external environment. The light scattering response characteristics of the sensor are calculated based on Mie scattering theory. The sensor performance has been tested with standard par5ticles of polystyrene latex, the results show that the sensor has high signal-to-noise ratio, counting efficiency, and size resolution.