Positron annihilation lifetime technique has been applied to study the microstructure change in steel after heat treatment. For this purpose, the specimens are heat treated in various manners; normalizing, annealing, hardening, and tempering. The obtained spectra were analyzed into two components using the PATFIT program. The lattice defects in the specimens were investigated by positron annihilation parameters. Meanwhile, the microhardness of specimens was measured with a vickers hardness test (Hv) after the heat treatment processes. With the aid of image analysis method, the phases in heat treated samples and ASTM grain size were determined. A correlation between the positron parameters and ASTM grain size, microhardness, and phases in percentage, was investigated. The two state trapping model have been applied and trapping parameters calculated and correlated to microstructure changes.