A high strength steel belt having an excellent flatness and a duplex structure of austenite and martensite has been prepared by a process which comprises providing a cold rolled or cold rolled and annealed strip of a martensitic structure from low carbon martensitic stainless steel containing from 10 to 17% by weight of Cr and having a carbon content of not exceeding 0.15% by weight, connecting ends of the strip or ends of a plate cut from said strip to provide an endless belt, causing the endless belt to circularly move between rolls under tension and to pass through a heating furnace where the belt is heated to a temperature within a range from (As point of the steel+30.degree. C.) to Af point of the steel and not higher than 900.degree. C. so that a part of the martensitic phase may be changed to a reversed austenitic phase and a desired surface flatness may be obtained after cooling.