A miniature optical sensor for laser particle counter is described, and some calculated and test results are reported in this paper. A reflective spherical mirror coated with highly reflective optical film is applied as collecting element for scattered light, and a PIN photodiode with high performance is used as the photo-detector. A band-pass preamplifier is used to eliminate lower-frequency electromagnetic interference from external environment, as well as to filter high-frequency components from electronic noise. An air sampling system can provide a very constant flow rate. The smallest particle diameter of the optical sensor is 0.3 microns with a signal-to-noise ratio exceeding 2:1.