The research on the distributions of metal powder stream from co-axial nozzle is very interested in laser manufacturing. The paper was focused in the concentration field of powder stream. Theoretic model of concentration field of powder stream from the co-axial nozzle has been established. A new digital particle image measurement technique has been developed to measure the concentration field of powder stream. The system mainly consists of Nd+:YAG laser and charge coupled device (CCD) camera. It is shown that focus parameters and concentration distribution of powder stream can be measured by the technique. The system could be a useful device for laser manufacturing after further developed.