A theoretical model is introduced to derive the ME voltage coefficients of bilayer in free state according to the constitutive equations of magnetostrictive and piezoelectric phases. Using this model, the ME voltage coefficients of La07Sr03MnO3(LSMO)-PZT and Tb1-xDyxFe2-y (TDF)-PZT bilayers have been calculated and analyzed using the corresponding material parameters of individual phases. The results show that the ME voltage coefficient increases to a maximum at a given volume fraction of piezoelectric phase. With increasing interface coupling parameter k, an approximately linear increase of the maximum ME voltage coefficient have been observed. The maximum ME voltage coefficient for TDF-PZT bilayer reaches 19Vcm-1Oe-1, while for LSMO-PZT bilayer, the maximum value is only 165mVcm-1Oe-1. The theoretical results of ME voltage coefficients versus bias magnetic field for LSMO-PZT bilayer have been found to agree well with the measured data when k=02. Analysis shows that large magnetostriction, appropriate volume fraction and adequate interface coupling are key facters for obtaining excellent ME performance.