The spray characteristics were investigated with and without gas perturbation by varying the momentum flux ratio and gap thickness.The film thickness was measured by the electrical conductance.The spray angle was obtained by 20 images after post processing.The gas disturbance was generated by a speaker and the response characteristic was expressed as the gain value.As the gap thickness influenced the loss of liquid momentum,the gas flow strongly affected the spray characteristics.As the gap thickness became thin,the film thickness and spray angle were small,and the gain was sensitive to the momentum flux ratio.Therefore,the injector could be weakened by combustion instability on certain frequency where the amplitude of gain has high value.