Aluminum alloy (A2024-T3) specimens were used for a fatigue testing by subjecting them to a stress amplitude of 150 MPa. At different numbers of the fatigue cycles, specimens were removed from the fatigue tester, and then subjected to the ultrasonic measurement, the dislocation density measurement, the hardness testing, and so on. From the Fourier spectrum of the bottom echo, the peak intensity (PI), the peak frequency (PF) and the average gradient of the transfer function (AGTF) were obtained. The dislocation density was obtained by the X-ray diffraction analysis. AGTF, the dislocation density and the hardness decreased at the initial stage, and then gradually increased with the increasing number of the fatigue cycles. PI showed a tendency to increase as fatigue cycles increased, but no change occurred in PF. The change of ultrasonic parameters in the fatigue process was quantitatively discussed according to Granato and Lücke dislocation-string model. Then the in-process ultrasonic measurement was carried out in the fatigue testing of the aluminum alloy by using the water bag to obtain ultrasonic parameters. PI and AGTF rapidly increased at the initial stage, and then gradually increased with increasing number of the fatigue cycles. These results suggest that the dislocation density steeply increased at the initial stage of the fatigue process, and then they gradually increased.