In this paper, visualization by means of the Schlieren method was accomplished in a two dimensional flow channel model of a CNG engine mixer. From the visualization results:(1)Mixing in the region of the venturi tube and throttle valve was influenced by the throttle opening and by the distance of the nozzle and valve, and in addition in this region natural gas behavior shows many different flow patterns.(2)The mixing (diffusion) characteristics clarified the relationship between the throttle opening and two natural gas flows; high velocity flow near the channel wall and swirl flow under the throttle valve.(3)The concept of gas and air mixing being affected by the dimensions of the main elements (main nozzle, venturi tube, throttle valve, their relative relationships and auxiliary air) of the CNG mixer were clearly shown.Premixing of natural gas and air in a CNG engine vehicle is said to be inadequate because it adversely influences the engine combustion and emission characteristics. It is evident that this subject has not had sufficient study in the past(1),(2). These references show the visualization with gas combustion. The mixing phenomenon was then observed from the visualization results. The channel model consisted of a main nozzle, throttle valve and manifold. The experimental parameters were the main nozzle distance, venturi tube diameter, throttle opening and mixture ratio. Observations showed the difficulty of mixing natural gas and air. It is concluded that a CNG mixer design with good mixing characteristics can be estimated from the results of this study.