The particle entrainment behaviors at the outlet of a nozzle in the gas particle coaxial jets are experimentally investigated by morphology. The particles in the surface of the granular stream discharging from an inner channel are entrained into the recirculation region due to the drag force of the gas backflow. Hence, the gas backflow is the most essential and crucial factor in the entrainment phenomena. Moreover, the experimental results demonstrate that the particle mass flow rate, the inner duct thickness and the particle size have influences on the formation as well as the characteristics of the entrainment phenomena. The particle entrainment critical annular gas velocity increases linearly with the initial particle velocity and is responsible for the inner duct thickness and the particle size. The entrainment length of the particles and the reverse dispersion angle are employed to analyze qualitatively and quantitatively the motions of the particles. Finally, two empirical correlations on the entrainment length of the particles and the reverse dispersion angle are achieved. (C) 2015 Elsevier B.V. All rights reserved.