Abstract: Longitudinal waves were used to measure the applied stress in LY11 Aluminum alloy. Experimental results showed that the presence of stress caused small changes in phase velocities of longitudinal waves, and the velocity was increased by increasing applied stress. Furthermore, the percentage change in velocity was proportional to the stress. This linear relationship provided a practical non-destructive technique of stress measurement. The phase velocity was tested by sing-around methods, which small changes in velocity could be detected.