The microstructures of 304 stainless steels with different amounts of nanocrystalline and microcrystalline austenite prepared by an aluminothermic reaction casting, without and with annealing at 1073 K for 8 h, have been investigated by X-ray diffraction, an electron probe micro-analyser, a transmission electron microscope and a scanning electron microscope. The steels, both without and with annealing, consisted of different dual nanocrystalline and microcrystalline austenite combinations and a little nanocrystalline δ ferrite, while the average grain size of the nanocrystalline austenite increased from 19 to 26 nm and volume fraction of the microcrystalline austenite increased from 17 to 30% after annealing. The tensile strength of the steel was dramatically increased from 500 to 1000 MPa and the tensile elongation ratio increased from 8 to 12% after annealing. However, the tensile strength was decreased to 600 MPa and the tensile elongation ratio increased from 12 to 22% after an annealing at 1273 K. The combination of dual nanocrystalline and microcrystalline austenite obtained after the annealing at 1073 K results in the best tensile properties.