<p>To detect the internal deterioration in the concrete covered with carbon fiber reinforcement polymer （CFRP） sheets, this study developed a non-destructive test using electrical impedance variation and phase transition. The study considered a hypothesis that the concrete exhibits inductive, capacitive, and resistance properties depending on the frequency and permittivity of the applied voltage. Deterioration damage, such as cracks inside the concrete, is a type of air layer that causes polarization. The magnitude of polarization is expressed by the relative permittivity. The relative permittivity of concrete is about 6-10, and the relative permittivity of air is 1. In the case of cracks, which are a type of air layer, occurring inside the concrete, the apparent relative permittivity decreases. Changes in the relative permittivity of concrete can be captured by measuring the impedance and phase angle. The study conducted a freeze-thaw test using CFRP-covered concrete specimens. Furthermore, the study examined the frequency characteristics of impedance and phase under AC voltage with frequency ranging from 1MHz to 8MHz. The impedance and phase were measured using a probe with two electrode terminals. These frequency properties were evaluated by an impedance analyzer, which was controlled by a computer. The test results confirmed that the capacitive phase property of damaged concrete altered to exhibit inductive characteristics in the range of 2 to 4MHz. The fundamental test revealed that the internal damages of the CFRP covered concrete are detected by measuring a frequency at the impedance local maximum value and the phase transition property.</p>