Abstract: The Semi-Analytical Finite Element approach is used for calculating dispersion curves of an annulus in the circumferential direction. As a result, the group velocities can be calculated independent of phase velocities. Phase velocities, group velocities and displacement fields of the lowest order mode of the circular aluminum annulus (Outer diameter: 202 mm, inner diameter: 200 mm) are calculated at the frequency of 2.5 MHz. In experiments, the waveform of thin-walled aluminum annuluses with and without the interior crack (Depth: 0.5 mm) is analyzed, then the group velocity is calculated by the time and distance of the backscattered signal in order to determine the mode. Mode conversion by the crack is detected and the interior crack in the aluminum annulus is located accurately by experimental data.