The mechanism of electric field (EF) effects on the magnetocrystalline anisotropy (MCA) in metallic films is investigated by first-principles calculations. Start with a simple system of Fe, Co and Ni monolayer on Cu(001) substrate, we show that the key factor for a large EF-induced MCA modification is that the energy bands cross of d3z2−r2 and dyz (or dxz and dxy) is close to the Fermi level. In order to enhance the MCA modification by EF, 4d metal substrates (Rh, Pd) are also discussed. In particular, we find that the magnetization direction can be switched from out-of-plane to in-plane by a small EF for Fe 1–x Co x alloy films on Rh(001) substrate with x=0.5.