An investigation of the dispersion mode of the granular jet in a coaxial air jet has been conducted. In all cases, the dispersion mode is dominated by the outer air jet. Three dispersion modes, namely, the shear dispersion (u(g) < 24 m/s), wave dispersion (24 m/s < u(g) < U-2) and oscillating dispersion (u(g) > U-2), are identified. U-2 increases with the granular jet mass flux and the particle diameter. The observed wavelength of the Kelvin-Helmholtz instability of the wave dispersion increases in the multi-phase flow field along the jet flow direction. The first wavelength and its location decreases strongly with the air jet velocity. And the occurrence frequency of the first wave increases with the granular jet mass flux and the air jet velocity. The oscillating frequency of the oscillating mode increases linearly with air jet velocity. The granular jet mass flux and the particle diameter also have a slight effect on the oscillating frequency. It is found that the Strouhal number, St, of the granular jet can be used to distinguish the dispersion mode of the granular jet, when 0.07 > St > 0.03, it belongs to the wave dispersion mode, when 0.02 > St > 0.01, it belongs to the oscillating mode, and when 0.03 > St > 0.02, it belongs to the transition state between the wave dispersion and the oscillating dispersion. (C) 2011 Elsevier B.V. All rights reserved.