Based on the empirical electron theory of solid and molecules (EET) the valence electron structures (VESs) of diamond, graphite, Fe3C and gamma-Fe crystal were constructed, and the relative electron density differences (REDDs) of the diamond growth interfaces were calculated. It was found that the REDDs of graphite/diamond interfaces were all great, while three among those of Fe3C/diamond interfaces were little enough that the electron densities were continuous at the first order of approximation, which means that the carbon atom groups are easier to decompose from Fe3C than from graphite and to transform into diamond structure. Moreover, two of the REDDs of gamma-Fe/Fe3C interfaces were continuous. Accordingly, it can be reasonably suggested that the carbon source for diamond crystal growth may come from the decomposition of Fe3C instead of graphite, gamma-Fe plays a role of catalysis to promote the decomposition of Fe3C, and the diamond transformation should be finished on the diamond/film interface. (c) 2009 Elsevier B.V. All rights reserved.