Self-pulsation usually occurs in the gas-liquid swirl coaxial injector of a liquid rocket engine and it is accompanied by an intense scream. When self-pulsation occurs, pressure and flow rate oscillations are detected by the strong interactions between the liquid and gas phases. Because this oscillation can amplify combustion instabilities in liquid rocket, self-pulsation must be suppressed. Considerable research has been performed on self-pulsation characteristics, but the mechanism of self-pulsation has not been clarified. The main objective of this research is to understand the mechanism of self-pulsation. Toward this objective, the acoustic and spray characteristics of self-pulsation spray were investigated using the acoustic device and laser system; the dominant frequency of the liquid sheet was also measured. It was found that the frequencies of the scream and the spray oscillation are linearly proportional to the liquid and gas axial Reynolds number, and the self-pulsation frequency is determined by the unstable wave of the liquid sheet. Therefore it is concluded that self-pulsation occurs due to the dominant wave of the liquid sheet.