GPR has great potential in investigating water leak and water seepage because of its sensitivity to water. Given water as a dominant factor affecting reflection strength, wave propagation velocity of radar and absorption of high frequency part in spectral content for the non-metallic substance, thus this feature can be used to characterize water leak and seepage. This paper studies the phenomenon of subsurface water leak and demonstrates the experimental effort of pinpointing the water leaks of a buried water mains via a field-scale water leakage experiment in a designated site area (20m long × 10m wide) by analyzing change in GPR's wave propagation velocity. In the site, a ductile iron (D.I.) pipe with four displaced joints were buried and overlaid by concrete and paving blocks. Results obtained by IDS antenna with nominal center frequency of 600MHz were used to study and validate the effects of water leakage by velocity analysis based on . The GPR wave propagation velocity analysis and imaging are based on the GPR data acquired before and after the water injection to map the location of leak points. With the reliable result attained from the velocity analysis, it is validated that the leak points of the pipe could be identified and verified by comparing the velocity profile and providing an accurate method for engineers to locate pipe leak.