Electromechanical coupling coefficient kt2 is one of the important parameters to estimate performance of BAW filters. The resonance-antiresonance method according to IEEE standard is common method to determine kt2. However, this method requires a self-standing film structure (FBAR). In this study, we propose the kt2 estimation method for a film/substrate structure (HBAR). This method allows us to estimate kt2 for as grown wafers without removing substrates. The lattice strains of piezoelectric crystals are induced when electric field are applied. The strains can be measured by X-ray diffraction (XRD). Therefore, the kt2 of the film is expected to be indirectly determined by the use of piezoelectric equations. The values of kt2 determined by this method were compared with the resonance-antiresonance method and the conversion loss method.