In this study, we examined how the materials’ properties of a lead magnesium niobate-lead titanate solid solution, [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 (PMN-PT) piezoelectric plate sensor (PEPS) affected the enhancement of the relative detection resonance frequency shift, −Δf/f of the sensor, where f and Δf were the resonance frequency and resonance frequency shift of the sensor, respectively. Specifically, the electromechanical coupling constant, −k31, of the PMN-PT PEPS was varied by changing the grain size of the piezoelectric layer as well as by applying a bias direct current electric field. Detection of streptavidin at the same concentration was carried out with biotin covalently immobilized on the surface of PEPS. It is shown that the −Δf/f of the same streptavidin detection was increased by more than 2-fold when the −k31 increased from 0.285 to 0.391.