We analyse the Hopfield factor of the newly found superconductor MgB2 using the linear muffin-tin orbital method. Based on a uniform transferred charge density model raised, it is shown from our calculation that the high difference of electronegativity between boron and magnesium favours the high l(l + 1) angular momentum hybridization and then the Hopfield factor. Our analysis is consistent with experimental results. Comparisons with cuprate superconductors are also discussed.