[en] In this work, we report a systemetic study of Co substitution for Ru in Ru_2FeSi. Our previous studies showed that Ru_2FeSi and Co_2FeSi are antiferromagnetic and ferromagnetic, respectively. In order to understand the influence of Co substitution for Ru, (Ru_1_−_xCo_x)_2FeSi alloys were prepared by arc melting, followed by annealing at 1273 K for 3 days. Structural and magnetic studies were carried out on the quaternary alloys by powder X-ray diffraction and vibrating sample magnetometry, respectively. At room temperature, Ru rich compositions i.e., x=0 and x=0.25 exhibited disordered B2 structure, but with increase in Co concentration L2_1 ordering appeared in the alloys. From magnetization measurements, it is seen that with increasing x, the ferromagnetic state becomes dominant with the appearance of spontaneous magnetization and increase in the value of magnetization of the alloys. Saturation magnetization measured at 15 kOe increased from 1.02 emu/g to 149.96 emu/g as x was increased from 0 to 1.