Aiming at elucidating the flow characteristics of liquid jet at a coaxial type injector, experimental visualization and theoretical analysis as well as numerical simulation were carried out. For computing atomization phenomena, a numerical method has been developed, and it was firstly verified through quantitative comparisons with corresponding experiment of pinch off. It was confirmed that the method can compute inertia force, interfacial tension, viscous force and gravity force adequately, all of which generally affect atomization phenomena. Then, it was experimentally confirmed that fast gas flow enhanced atomization of liquid jet. Numerical analysis showed satisfactorily good agreement with corresponding experimental results. When the gaseous injection velocity became slow at constant mass flow rate, the atomization was suppressed, which was coincident with linear stability analysis of two dimensional liquid/gas parallel flow. It was clearly represented that installation of recess enhanced atomization due to straight gas flow guided by the recess. When the fast gas blew close to liquid jet, the amplitude of instability wave grew, and, additionally, impact of gas on liquid resulted in promotion of atomization.