Recently, the demand for fine particles of pm or sub-pin-order has risen strongly in various industrial fields. In many powder-manufacturing processes, a supersonic jet mill is used for the pulverization of various kinds of fine particles. The improvement of pulverization efficiency is very important, because the jet mill consumes a large amount of operating power. In this study, a new supersonic jet nozzle with a rectangular cross section was proposed, and its flow characteristics and the particle acceleration performance were investigated experimentally by measurement of particle velocity and flow visualization by the Schlieren method. Numerical analysis was also performed for reference. As a result, it was found that the new supersonic jet nozzle has a high performance for particle acceleration, namely, for improvement of pulverization efficiency.