This time, we would like to report our recent study on Pb(ZrTi)O3(PZT)-based ferroelectrics, currently one of interesting topics in condensed matter science. In this study, a new method, called synchrotron X-ray microdiffraction (XRMD) in situ, was introduced to examine the electric field-induced structural modulation of the epitaxially grown tetragonal PZT thin film. To evaluate the d-spacing (d001) from the measured intensity contour in the two theta-chi space, the peak position in each diffraction profile was determined by applying the two-dimensional Lorentzian fitting. By tracing the change of d-spacing as a function of the applied electric field and by examining the Landau free energy function for P4mm symmetry, we were able to estimate the two important parameters that characterize the field-induced structural modulation. Further theoretical analysis shows that the compressive epitaxial in-plane stress dominantly contributes to the elongation of the c-axis lattice constant in the c-axis oriented epitaxial PZT film.