Various methods of nondestructive testing offer the opportunity to understand hydration of cement-based materials. In particular, the ultrasonic wave reflection method does not require access to two sides of a structural member and can be performed continuously and on site. This advantage is very significant when estimating the in situ compressive strength of early-age concrete. However, it is also necessary to understand which properties are measured by the ultrasonic wave reflection method and compare the measurements with other testing methods. This paper demonstrates the nature of these measurements with the wave reflection method by comparing the results of the other nondestructive testing method using resonant frequencies.