Reaction fin is a device for improving propulsive performance of ships by providing preswirl to the propeller inflow for cancellation of rotation in the slipstream. This report describes the development of reaction fin for high block coefficient ships undertaken in Mitsubishi Heavy Industries, Ltd. First, explanations are given on the effectiveness of reaction fin for full ships with some results from basic researches carried out in the early stage. Next, development made on its practical application is described which includes studies on various aspects such as structure, vibration, monouevrability etc. as well as propulsive performance. Finally, full-scale results are introduced from the results of the two ships fitted. with a reaction fin, showing that it is as effective as expected for improving propulsive performance without causing any troubles for service.