Microcomposite particles Sr3Ti2O7-SrTiO3 with an epitaxial core-shell structure were prepared by the hydrothermal treatment of Sr3Ti2O7 core particles. The Sr3Ti2O7 core particles synthesized in a fused salt had a plate-like morphology with a well developed {001} plane of the Ruddlesden-Popper type crystal structure. The particles after hydrothermal treatment have an anisotropic morphology and a core-shell microcomposite structure : the plate-like Sr3Ti2O7 particles covered with SrTiO3 fine particulates and thin layers. TEM analysis revealed that the SrTiO3 shell on a Sr3Ti2O7 core particle had an epitaxial relationship with the core such that {100} SrTiO3//{001} Sr3Ti2O7 and [010] SrTiO3//[010] Sr3Ti2O7. The possible formation mechanisms of the SrTiO3 phase were through solution-precipitation and the extraction of Sr-O layers from the Sr3Ti2O7 structure under the hydrothermal conditions. The processing method will be extended to the other type of microcomposite with an anisotropic morphology and an functional surface phase.