SrBi2Nb2O9 thin films were deposited on (001) SrTiO3 substrate by sol–gel spin coating. A previous study showed that the film crystallizes with the c-axis normal to the surface. Those epitaxial films are studied by means of X-ray diffraction (XRD) line profile analysis as a function of thermal annealing duration. The line profile analysis of the diffraction patterns collected in ω-2θ scan mode, gives detailed information on the coherently diffracting domain size and microstrains along a given direction. For low annealing duration the width of the (001) diffraction lines reaches values of approximately 1°. In accordance with a recent study, integral breadth and Fourier analysis suggest the presence of stacking faults separated by a mean distance of 5 nm. The profiles exhibit a marked Lorentzian character as expected from a faulted crystal. In addition to faulting, both finite grain size and microstrains contribute to the observed width. When heat treatment time is increased, the breadth and Lorentzian content of the (001) diffraction lines decrease attesting that the stacking fault density is lowered. For a 500-h treatment at 700°C the calculated domain size equals the films thickness. This indicates that stacking faults have almost disappeared: the SBN crystallites of the film have reached an equilibrium state.