Abstract: Dispersion curves of free bar and steel bar embedded in the soil were analyzed. With a testing system, guided wave testing was carried out on the rock bolts which was free and embedded in the soil to inspect the length of rock bolts. Results showed that dispersion might direct the guided waves testing in theory; a suitable frequency range was chosen by analyzing dispersion curves and it was effective for inspecting the length of rock bolt using axisymmetric L(0,1) mode of ultrasonic guided waves. Compared to the rock bolts in the air, the amplitude of signal received from the rock bolt end embedded in the soil was damped. Therefore the defect in bolt embedded in soil could be detected by L(0,1).