We investigated the martensitic transition and the magnetic properties of Mn1−xCoGe melt-spun ribbons. The as-prepared Mn1−xCoGe ribbons crystallize in austenite hexagonal phase with a textured structure. The postannealing process promotes the formation of the martensitic phase and homogenization of the alloy, resulting in a first-order magnetostructural transition in the annealed ribbons, and thus a giant magnetocaloric effect. The magnetic entropy change around the transition reaches 19 J/kgK for a magnetic field change of 0–5 T. Furthermore, it is found that the hysteresis loss around the magnetostructural transition is negligible in present annealed ribbons, which would facilitate the application of Mn1−xCoGe alloys.