The effect of various substrates on the electrical and electromechanical properties of 100-nm-thick epitaxial 0.9[Pb(Mg1/3Nb2/3)O3]–0.1[PbTiO3](0.9PMN–0.1PT) thin films is investigated. (001) 0.9PMN–0.1PT films are grown on (001)LaAlO3(LAO), (La, Sr)(Al, Ta)O3(LSAT), SrTiO3(STO), and MgO substrates with 40-nm-thick top and bottom La0.5Sr0.5CoO3 electrodes by pulsed laser deposition. X-ray diffraction results indicate that the films on LAO, LSAT, and STO are stressed biaxially in compression in the film-substrate interface whereas the films on MgO are stressed in tension. A decrease in the temperature of dielectric maximum (Tm) together with an increase in the dielectric constant and the longitudinal piezomodulus is observed with decreasing in-plane epitaxial stresses for LAO, LSAT, and STO substrates. The films on MgO substrates have the highest dielectric constant and piezomodulus with Tm below room temperature. The variation in Tm may be attributed to the shift in the transformation temperature from the paraelectric state to the relaxor state due to internal stresses in the film-substrate interface. Electrical and electromechanical properties should depend strongly on internal stresses in the vicinity of the phase transformation, which is reflected in our experimental observations. © 2000 American Institute of Physics.