Aluminium alloy (A2024-T3) plates with curved sides were subjected to the low-cycle fatigue testing under conditions of a stress ratio of 0.05 and a frequency of 0.001 Hz. At an early stage of the fatigue process, the ultrasonic wave was measured. A new real-time measurement method of local immersion was developed by using a water bag to measure the ultrasonic wave. Measured wave was analyzed through fast Fourier transformation (FFT) to obtain the ultrasonic parameters, such as the peak intensity and the average gradient of the transfer function. Relations between the measured ultrasonic parameters and the measured local strain were examined during one cycle in the early stage of low-cycle fatigue testing. In one cyclic loading, the propagation time of the ultrasonic wave from the surface to the bottom of the specimen sinusoidally changed with time corresponding to the strain change of the specimen. The peak intensity and the average gradient of the transfer function of the bottom echo tended to sinusoidal change with time during arbitrary one cycle. The influences were discussed of the acoustoelastic velocity change and the change in the focal position of the transducer following deformation of the specimen on the propagation time of the ultrasonic wave.