Element substitution is usually used to improve the glass forming ability (GFA) and magnetic properties of alloys.We obtain Gd55Al20Ni20Co5 bulk metallic glass (BMG) by minor Co substitution for Ni in the Gd55Al20Ni25 glass-forming alloy.The Gd55Al20Ni20Co5 BMG exhibits a better GFA and magnetocaloric effect (MCE) than the Gd55Al20Ni25 BMG.The mechanism for the enhanced magnetic entropy change and refrigeration capacity of the Gd55Al20Ni20Co5 BMG is investigated.