A general graphical proof for the second Carnot principle is presented using the temperature–entropy diagram. This proof is simple and is independent of the proof of the first Carnot principle. The Carnot relation for thermal efficiency is also obtained simultaneously for all totally reversible heat engines between the same two reservoirs. The concept of reversibility in heat transfers and the role of environments in heat engine cycles will be further clarified and could be better understood using the proof presented here, which will be useful both for teaching and performing research in thermodynamics.