In order to study the nonlinearity during the start-oscillation of thermo-acoustic instability, an experimental setup was built. The growing process of nonlinearity during the start-oscillation of thermo-acoustic instability was captured and analyzed. Experimental results revealed that after a suitable resonance mode corresponding to the structural of the combustor was selected, the pressure perturbations inside the combustor grow in amplitude into a very large amplitude and self-excited oscillation in a very short period of time. Then, slowly, the nonlinear effects adjust the shapes of pressure waveforms and amplify the oscillations. Ultimately, a limit-cycle oscillation with smooth and uniform pressure waveforms was obtained, and the acoustic waves exhibit only the main resonance mode, damping other modes of instability.