ABSTRACT In this study, the effects of complex phase(CP) microstructures obtained by applying various heat treatment routes on the impact toughness properties of unalloyed high carbon powder metallurgy (PM) steels containing 1.2% carbon by weight were investigated. For this purpose, after applying quenching and tempering heat treatment to the sintered samples, they were partially austenitised at 800°C for 5 min, cooled in water (γ + Fe3C), and tempered for 1 h. The mechanical performances of CP microstructures were compared with sintered, quenched, and conventional quenched tempered specimens, respectively. The crystallographic analyses and dislocation densities of the samples, which were microstructurally characterised by SEM and TEM analyses, were determined by the XRD device. The hardness and impact toughness values of the samples with CP microstructures were improved by 64% and 23%, respectively, compared to the initial sample.