A submicro-steel sheet was successfully fabricated by severe warm-rolling at 773 K through a single pass. The microstructure was characterized first and the thermal stability of the submicro-steel was investigated by annealing the steel at different temperatures. Results indicate that grains are nearly equiaxial with an average diameter of 300 nm for grains near to the surface and 600 nm for grains at the center of the sheet and the submicro-steel can be subjected to annealing at 773 K without obvious grain growth. The formation of the submicro-structure can be related to a severe plastic deformation-induced grain refinement mechanism. The unusually high thermal stability can be attributed to the pinning effect of numerous uniformly-distributed nano-precipitates in the steel. The average diameter of the larger precipitates is about 30 nm and the smaller less than 10 nm. A microsmicro-steel sheet was successfully fabricated by severe warm-rolling at 773 K through a single pass. The microstructure was characterized first and the thermal stability of the submicro-steel was investigated by annealing the steel at different temperatures. Results indicate that grains are nearly equiaxial with an average diameter of 300 nm for grains near to the surface and 600 nm for grains at the center of the sheet and the submicro-steel can be subjected to annealing at 773 K without obvious grain growth. The formation of the submicro- The unusually high thermal stability can be attributed to the pinning effect of substantially uniformly distributed nano-precipitates in the steel. The average diameter of the larger precipitates is about 30 nm and the smaller less than 10 nm.