The effect of Gd diffusion on structural evolution and superconducting properties of YBa2Cu3O7-x, (Y123) prepared in the bulk polycrystalline form were studied. The gadolinium (Gd) diffusion in superconducting Y123 ceramic has been studied over the temperature range of 600-900 degrees C by energy dispersive X-ray fluorescence technique (EDXRF). The temperature dependence of Gd diffusion coefficient in grains (D-1) and over the grain boundaries (D-2) are described by the relations: D-1 = 8.9 x 10(-6) exp(-1.25/kT) and D-2 = 2.3 x 10(-5) exp(-1.091U). For the Gd diffused-doped samples, magnetization and resistivity measurements show that the critical transition temperature, T-c increased from 88 to 91 K and the critical current density, J(c) which was calculated from M-H loops taken at 77 K, increased from 55 to 122 A cm(-2) in comparison with those of undoped Y123. The possible reasons for the observed increases in T-c and J(c) due to Gd diffusion were discussed. In addition, magnetic hysteresis curves of the Gd900, Gd800, Gd700 and undoped Y123 are presented for temperature of 5 K. It was found that the hysteresis is enhanced by the Gd diffusion. Such enhancement, which is considered to represent a characteristic strength of intergrain coupling, is more clearly recognized when critical current densities are compared. (c) 2006 Elsevier B.V. All rights reserved.