Recent development of ab initio calculations makes it possible to use the total energy difference between pure element metastable structure and standard element structure for phase diagram construction also for complex intermetallic phases. The physical model of sigma-phase is proposed and tested on the Fe-Cr system, using data from literature for remaining phases. The model is based on the two sublattice model as for solid solution phase, but structure energy difference for pure components is based on the results of calculations of electronic structure from the first principles. Entropy part of Gibbs energy of elements in sigma-phase structure and excess Gibbs energy of mixing have to be still adjusted to phase equilibrium data.