An ideal density of dangling bonds and the surface free energy (SFE) of (1×1)-(hkl) surface of cubic crystals are calculated by using a broken-bond model. The results show that the SFE(γ) can be expressed as γ=f(hkl)·(Eb/d2 0) where f(hkl) is a periodically convergent function, Eb and d0 are respectively the bond energy and the bond length. The f(hkl) is rela ted to the crystal structure. The anisotropy of the SFE and the equilibrium form (EF) of the crystal can be readily determined by using the results. It is found that the EFs of the crystals of fcc and bcc are truncated octahedron and rhombi c dodecahedron, respectively, which are coincident with the corresponding crysta ls' three dimensional first Brillouin zones, respectively.