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A novel Fe-28Mn-10Al-C-0.5Nb low density steel containing 0.5wt% Nb for automobile was developed, aiming to study the existence of Nb and the effect of Nb addition on the structure and mechanical properties of austenitic Fe-Mn-Al-C low-density steel. The results show that, Nb is distributed in austenite grains and grain boundaries in Fe-28Mn-10Al-C-0.5Nb low density steel in the form of NbC. With the addition of Nb, the average austenite grain size in Fe-Mn-Al-C low density steel is refined from 39.49μm to 13.67μm，and the formation of annealing twins is also suppressed by the addition of Nb. With the addition of Nb, the yield strength and ultimate tensile strength of Fe-28Mn-10Al-C low density steel increased by 64-170 MPa to the other, and the elongation after fracture increased by 11%, revealing an excellent balance of strength and ductility. This is due to the precipitation strengthening of NbC precipitates and fine grain strengthening of austenite. Precision strengthening plays main role to the increase of yield strength of Nb-added low density steel.