The flow and mixing process of unsteady jets are fundamentally analyzed by using large eddy simulation (LES). The effects of nozzle velocity and turbulence intensity on the turbulent eddy structure and mixing process between nozzle fluid and ambient fluid were investigated. The results show that toroidal-shape vortex, which is growing up around jet tip, mainly accelerate the entraining flow. Also, increasing the turbulence intensity in the nozzle encourages the mixing in the jet without change of the jet-contour. Furthermore, when the rise-up time of initial nozzle velocity is elongated, turbulent mixing is suppressed.