[en] Electronic properties including magnetoresistance have been investigated for polycrystals of ordered double perovskite alloy, Sr₂Fe(W_1-xMo_x)O₆. In this system, the ground state changes from an antiferromagnetic insulator to a ferromagnetic metal with increasing x. The magnetization in Sr₂Fe(W_1-xMo_x)O₆ develops by the minimal substitution of W by Mo, and then the conductivity as well as the density of states at the Fermi level grows rapidly with x above 0.25. The present alloy also shows a variety of magnetoresistance behaviors. For example, Sr₂Fe(W_0.4Mo_0.6)O₆ (x=0.6) exhibits a magnetoresistance (MR) at 4.2 and 300 K as large as 69 and 10%, respectively. Furthermore, Sr₂Fe(W_0.85Mo_0.15)O₆ (x=0.15), located near the insulator-metal phase boundary, shows an extremely large magnetoresistance as large as 195% at 16 K. The low-field MR can be ascribed to a field suppression of the spin-dependent scattering at the grain or domain boundaries