Abstract: Effect of aging treatment on impact toughness of MFT8 non-quenched and tempered steel for fastener wire with 28% in draft was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile and impact testing method. The results show that the impact toughness of the test steel increased first then decreased with the increase of aging temperature. In comparison with the original sample, the sample aged at the best temperature of 300 ℃ for 2 hours had the best impact toughness, which increased by 8.7%. This increase is attributed to the decrease of dislocation density and the formation of sub-microstructure with small size after aging.