The Yb1-xGdxBa2Cu3O7-y superconducting samples (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) which were prepared by solid-state reaction technique were studied to investigate the effect of x on c-lattice parameter, the activation energy, and the magnetic power constant of activation energy. c-Lattice parameters of the samples were calculated using XRD data and the activation energies of samples using the Arrhenius activation energy law were determined from the resistivity curves, obtained under six different magnetic fields up to 5 T in the range of temperature from 70 to 100 K steps by 0.25 K under zero cooling regime. The magnetic power constants of activation energies of the samples under different magnetic fields were calculated. The results showed that the value of c-lattice parameter is proportional to the size of ionic radius of atom, replaced by Yb in Yb123 structure. With increasing the x ratio, the activation energy and its magnetic power constant decreased and c-lattice parameter increased almost linearly. It is well known that when the magnetic field increases, activation energy of superconductor samples decreases. The control of activation energy of Yb1-xGdxBa2Cu3O7-y. sample using by x value and applied magnetic field can be useful in technical applications in superconductivity industry. (C) 2007 Elsevier B.V. All rights reserved.