There is provided a ferritic lightweight steel which contains 2.0 to 3.0 wt % manganese (Mn), 5.0 to 6.0 wt % aluminum (Al) and 0.1 to 0.3 wt % carbon (C) and has a tensile strength of 900 MPa to 1,108 MPa. The lightweight steel includes ferrite-austenite dual grains as a result of performing low-temperature tempering-induced partitioning (LTP) at 300° C. for 10 minutes after isothermal annealing.