A simple optical instrument is developed to measure remotely the size as well as the velocity of individual particles suspended in a fast-moving stream. The instrument consists of dual laser beams, each directed toward one phototransistor. The size and velocity are determined from the signals produced by particles passing through the laser beams. The measuring range of the particle size of the present unit is between 30 microm and 800 microm; both the upper and the lower ranges, however, are adjustable.