In recent years, failures of several earth retaining structures caused severe damage in property and loss of life in Taiwan. Therefore, evaluating the safety condition of existing earth retaining structures becomes an important task for the civil engineers. The objective of this research is to investigate the feasibility of applying Sonic Echo non-destructive testing technique for the evaluation of the height of existing cantilever earth retaining structures. Numerical modeling was performed to study the 3-dimensional (3D) effects on the measured signal. Field tests were also performed on two earth retaining walls to verify the correctness of numerical modeling. Results of this study indicate that, in comparison to 1-dimensional structure such as piles, 3D effects reduce the intensity of signal reflected from the bottom of the structure, thus increase difficulty in determining the arrival time of the reflected wave. However, the error resulting from these 3D effects is less than 10%. Therefore, it is concluded that the sonic echo method is a promising technique for the evaluation of the height of existing earth retaining structures.