The electronic structure and magnetic properties of Fe-based binary half-Heusler alloys Fe 2 Z (Z=In, Sn, Sb and As) have been studied. It is found that binary Fe 2 Z has a site preference similar to normal ternary ones. Hybridization between the d states of Fe (A) and Fe (B) leads to the formation of a d–d band gap near E F . This gap is broadened and shifted to low energy as Z varies from In to Sb, which makes Fe 2 Sb a half-metal. The half-metallicity of Fe 2 Sb is insensitive to the lattice distortion. The spin polarization ratio is always 100% from 5.6Å to 6.2Å. The spin polarization of Fe 2 As is also high at equilibrium lattice constant. With a small expansion of the lattice, it becomes an ideal half-metal. Fe 2 Z alloys are all ferromagnets with parallel aligned Fe spin moments.