Various research activities on combustion instability that significantly influences engine efficiency have been conducted. The instability of liquid fuel flow by perturbation of gas oxidizer flow in gas-centered swirl coaxial injectors is investigated using air and water. Gas perturbation is generated by a speaker, and variation in the film thickness at the end of the injector is measured. The spray patterns have different shapes, depending on the excitation frequency. The excitation effect is enhanced by the increment of the momentum flux ratio. Different gain values are achieved according to the input frequency, and a high value is realized in both low and high frequencies. The high- and low-frequency peaks are due to the resonant frequency of the gas feeding line and gas–liquid interface instability, respectively. The high-gain frequency can possibly cause combustion instability.