A low-density high-Mn test steel Fe-30Mn-9Al-0.96Mo-0.9C-0.49Si for cryogenic application was designed, and the effect of grain sizes on the microstructure and mechanical properties of the steel was investigated by tensile test at -196 ^oC, in terms of the microstructure evolution. The grain orientations of this steel are randomly distributed, and after tensile deformation at -196 ^oC the majority of the grain orientations tends to be <001>// the tensile direction or <111>// the tensile direction. Regardless of room temperature and -196 ^oC, the yield strength can be increased by approximately 160 MPa by refining its grain size from (16.5 ± 11.6) μm to (3.4 ± 2.2) μm. Accordingly, the steel after grain refinement presents yield strength, tensile strength and total elongation as 1304 MPa, 1664 MPa and 31.9% at -196 ^oC, respectively. Although the stacking fault energy can be reduced by lowering deformation temperature, the ordered structure still makes the plastic deformation of the steel to still be controlled by softening of slip planes. Thus, the plastic deformation mechanism of the steel is still governed by the planar slip. However, the dynamic slip band refinement and interaction between the slip bands can be affected by grain size.