Abstract: Borehole elasticity modulus measurements were conduct ed in five holes drilled from the excavated rock foundation surface of Xiaowan H ydropower Stations arch dam, situated on Lancang River, in order to evaluate t he unloading relaxation depth due to excavation blasting. The principle of boreh ole elasticity modulus measurement and results are presented, and the unloading relaxation depth is estimated. The depth is compared with that obtained by singl e -hole sonic velocity monitoring.