We fabricated Y1−xRExBa2Cu3O7−y (Y1−xRExBCO; RE=Gd, Sm) films on SrTiO3 (STO) single crystal substrates by the advanced TFA-MOD method. The Y1−xRExBCO films were found to be solid solutions between YBCO and REBCO by X-ray diffraction measurements and transmission electron microscopy (TEM). Jc–B characteristics of the Y1−xRExBCO films had been improved slightly in a high magnetic field than that of the YBCO film deposited by the advanced TFA-MOD. It was thought that swinging of the RE density in the solid solution played some role as a pinning center in the Y1−xRExBCO film under the high magnetic field. TEM observation revealed that RE elements are enriched in the vicinity of the interface between the film and the substrate. We suggest that RE-rich Y1−xRExBCO crystals easily nucleate and be epitaxially grown considering from the viewpoint of lattice matching to the STO substrate.