Thermal barrier coating (TBC) is applied to high temperature conditions in gas turbines, such as rotor blades and combustors, to protect the metal substrate surface against hot gas flow. The surface roughness of TBC is reported to affect the heat transfer coefficient, so surface roughness measurement is necessary for improving the thermal analysis of TBS. In this report, terahertz waves were used to measure the surface roughness of TBC specimens. An analytical one-dimensional model based on time of flight was constructed to confirm that the reflection characteristics of a terahertz wave pulse generated by a photoconductive antenna obey the Debye-Waller factor. The surface roughness of five TBC specimens fabricated by the same process was measured to be 16±2µm, which was in agreement with the microscopic observation result of the surface profile of a TBC specimen fabricated in the same lot. These results showed that terahertz waves are effective for measuring the surface roughness of TBC.