The element Co was added in the bulk glass-forming alloy Mg65Cu25Y10 to substitute Cu and to form Mg65Cu25xCoxY10 (x=2.5, 5, 7.5) alloys. Metallic glass strips with different thickness were prepared by a method of melt-injection casting into copper mold. Maximum thickness of complete amorphous structure was determined for the alloys. Amorphous nature of as-cast samples and crystallized phases during heating were analyzed with x-ray diffraction. The glass transition, crystallization and melting behavior of the alloys were investigated using differential scanning calorimetry. For the alloys with the Co content below 5 at. %, the glass-forming ability is comparable with the Co-free alloy. Metallic glass strip with a thickness of 2 mm can be attained. On further increasing the Co content up to 7.5 at, %, the glass-forming ability is weakened. Crystallization of the Co-containing metallic glass proceeds through a more complex process than that of the Mg65Cu25Y10 glass. With respect to the Mg65Cu25Y10 metallic glass, the partial replacement of the Cu by element Co yields a reduction of the supercooled liquid region AT, but there is no significant change in the reduced glass transition temperature T-rg.