We have studied the electrical and magnetic properties of RuGd_2–xCe_xSr₂Cu₂O_10–δ (Ru‐1222) with content of Ce = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, prepared by the standard solid‐state reaction technique. To determine how the magnetic and superconducting properties of this layered cuprate system can be affected by Ce substitution, the resistivity and magnetoresistivity have been measured at various temperatures. Resistivity curves reveal the presence of four regions for various contents of x. In high temperatures, system is paramagnetic. Magnetic transition from paramagnetic state to antiferromagnetic state occurs at 140 K for x = 0, which varies very slowly with increasing Ce substitution for Gd. In lower temperatures, second magnetic transition from antiferromagnetic to ferromagnetic is observed, which shifts from 60 K to 120 K with increasing x from 0.0 to 0.5. This variation may be caused by different ionic radii of Ce and Gd in this material which affects the Ru‐Ru interaction. Finally, the insulator‐superconductor transition take place at T_c, which shifts to higher temperature with appropriate variation of Gd/Ce. Superconductivity in the CuO₂ planes and weak ferromagnetism in Ru sublattice coexist and two states are practically decoupled. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)