The non-destructive testing based on stress waves has been used successfully for the determination of mechanical properties of hardened cement pastes and mortar. The transition period for these cementitious materials during the early age of hydration from viscous-elastic behavior to elastic solid state usually displays a continuous change in the physical properties with time for several hours after mixing. The ability of compressive waves travelling in solids generally is reliable, but relatively insensitive for the cementitious materials at early age hydration. However, shear waves only propagate in solids not liquid and cause the apparent energy loss while passing through viscous-elastic cementitious materials. Based on this wave characteristic, some acoustic wave methods have been developed to observe the process of setting in cementitious materials by measuring the reflection loss of shear waves. The reflection loss is linearly related to the strength gain of cementitious materials at early ages. An ultrasonic technique, which measures the reflection of shear ultrasonic waves at a cementitious material-metal interface, provides a continuous monitoring on cementitious materials with 3 different water-to-cement ratios, 0.25, 0.3, and 0.4, from very early age. The time of initial setting and final setting in cementitious materials are defined by comparing with micro-hardness testing. The measured reflection waveforms change with time for the change of acoustic impendence of stiffening cementitious materials. The continuously changing impendence in cementitious materials will be clearly affected on the cementitious material-metal interface changing. The first few waveforms with higher amplitudes at mixing gradually decrease at initial or final setting points for all mixtures. These findings are physically consistent with the shear wave characterization, a continuously decreasing reflection rate measured in hardening viscous-elastic materials. The initial or final setting points observed by micro-hardness can also be identified with shear ultrasonic wave technique.