The transformation behavior of a high carbon alloy steel A (SAE 52100) (1.02wt%C, 0.21wt%Si, 0.35wt%Mn, 1.44wt%Cr, 0.20wt%Ni) was investigated and compared to another steel B (0.53wt%C, 0.27wt%Si, 0.35wt%Mn and 0.66wt%Cr). It is shown that both the carbon and chromium contents in the super-cooled austenite increase with an increase in the austenitizing temperature. At a typical commercial heating temperature (840degreesC), the carbon content of the austenite in steel A is much lower than that indicated by the phase diagram, only reaching the level of a medium carbon steel. It is believed that the above result can be attributed to the inhomogeneous distribution of carbon and chromium in the unsaturated austenite. It is shown that the carbon content measured by an extraction method in austenite for steel A is only an average content of carbon, rather than the precise values in the different regions. Hence, for similar carbon and alloying element contents, the austenite in steel A is easier to transform than in steel B since the inhomogeneous carbon distribution in the austenite of steel A changes the nature of the transformation-temperature-time (TTT) diagram. These modifications to the TTT diagram promote the formation of pearlite with the proeutectoid Fe3C in the high carbon region, and bainite with the procutectoid alpha-Fe in the low carbon region of austenite, when the commercial quenching and low temperature tempering process was applied. This explains why some high carbon alloy steels have good overall mechanical properties.