The mixing characteristics of a coaxial round jet having inner to outer diameter ratio di/do = 0.67 was investigated to establish the method for mass and energy transport with inhibiting its diffusion. In the present study, the effect of the velocity ratio on the streamwise length of the potential core of the central round jet, the streamwise velocity distribution, and the evolution of vortcies of a coaxial jet was evaluated by using Large Eddy Simulation and experimental flow visualization to clarify diffusion inhibition mechanism of a central jet. Three velocity ratios γ = 0.25, 0.5, and 0.75 were conducted under a single condition of the central jet Reynolds number of 2000. The results show that the potential core length of a central jet become the longest on condition of the velocity ratio of 0.5, and the length was 15 times of the inner nozzle diameter. This value is equivalent to 3.8 times in the case of a general round jet. In other velocity ratio, it was also confirmed that the potential core length of a central jet becomes short by enhancement of vortices generation in the shear layer of annular and central jet. It is concluded that the velocity gradient in the shear layer is the important parameter in determining the diffusion of a central jet.