As a basic flow form, free round turbulent jet has been used in various industry fields such as water jet cutting,water cleaning, fuel spray, etc. To understand the development of jet and some of its underlying characteristics, it is crucial that the mechanism of jet instability should be recognized. A robust two-phase flow large eddy simulation (LES) algorithm was applied to predict the liquid instability and the coherent structure of free round turbulent jet. The main objective was to characterize the liquid instability mechanism without resorting stability analysis. Special attention was focused on how the instability and the eddy structure in the near field of round jet were affected by the imposed inlet velocity and disturbance. In this respect, the cases within the range of Re =10 000~ 80 000 at various disturbance intensities were tested. Results revealed that the velocity and disturbances played an important role in the stability of liquid jet. The unperturbed liquid core length and scale of surface wave were decreased, and a wide variety of vortexes were produced in the near field of jet with velocity and disturbance intensity increased. The decay rates of average velocity along the central axis of jet were similar under different disturbance intensities at the same Reynolds number (or different Reynolds numbers at the same disturbance intensity). The positive entrainment was enhanced and the radial spread of the jet along the streamwise became more significant with the Reynolds number increased.