To improve the understanding of dilution processes of ocean outfall in the surface-impinging region, a array detection system based on laser-induced fluorescence was employed to investigate the concentration field of a vertical round buoyant jet. From the experimental results, the empirical formula of axial and surface dilution was obtained for the pure momentum jet and buoyant jet. In the fully developed region of pure or buoyant jets, the axial dilution will increase linearly with dimensionless longitudinal distance except that, when the buoyant effects dominating the flow field, the tendency of dilution increase would be different. As the jet fluid entering the impinging region, the concentration field shows a characteristic of uniform distribution for both of pure and buoyant jets. From the comparison of buoyant jet dilution, it shows that the buoyant effect play a positive role in improving the surface dilution.