The effect of quenching temperature and tempering time on the microstructure, content of retained austenite and carbon content in retained austenite of a copper and niobium alloyed medium carbon steel were investigated by using optical, electron microscopy and X-ray diffraction. Results showed the microstructure of the quenching–partitioning–tempering steel consisted of lath martensite, retained austenite and carbides. With the increase of quenching temperature, the martensite laths became coarsened and the content of retained austenite increased. With the increase of tempering time, the content of retained austenite initially increased, reached to the maximum at 600 s and then gradually decreased.