The band structures and partial density of states of MM'X half-Heusler compounds as topological insulators have been simulated by using local density approximate + U. Results show that both the covalent M'-X bonds and the ionic M-M' bands contribute the band structures, and the covalent bonds between M' and X atoms decide the number of (E-Gamma 6 - E-Gamma 8) (Gamma(6) and Gamma(8) denote the twofold s-type orbitals and fourfold p-type orbitals and M, M', and X show atoms located at different positions in the lattice); big size of |chi M' -chi x| and small value of V are propitious to form topological insulators. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.