The improvement of mechanical properties of steels is an important priority for industry. Among different strengthening approaches, grain refinement is the only method to improve both strength and toughness simultaneously. Martensite process including cold rolling and subsequent annealing of the microstructures with high percentage of martensite is one of the effective methods for grain refinement in carbon steels. In this research, this process was applied to the low alloy steel DIN1.5715. Brittle martensite microstructures resulted in transverse cracks during rolling. Therefore, bainite microstructure with the same formation mechanism, could be the best candidate to replace the martensite. Several specimens were heat treated to form high percentage of martensite and bainite microstructures. Colour metallography was performed for determination of type and precentage of phases before rolling. The heat treated specimens were cold rolled with 90% reduction in thickness and then annealed at different times and temperatures. Microstructure evolution was characterized using optical and scanning electron microscopes at the each stage of the process. Mechanical properties of specimens were determined by hardness and uniaxial tensile test. Microstructure of the annealed specimens in both martensite and bainite processes revealed a high fraction of ultrafine ferrite grains with 100 nm and 300 nm, respectively. Mechanical properties of the ultrafine grain microstructures in the martensite and bainite processes showed tensile strength of 1200MPa and 700MPa, respectively. This indicated sixfold and threefold improvement compared with the initial structures. The elongation of the ultrafine grained steel in the bainite process (~20%) was twice of the one produced in martensite process (~9%). Keywords Thermomechanical process, martensite process, bainite process, cold rolling, annealing.