The site preference of alloy elements on sublattice is of both fundamental and technical importance in computational materials science.For example, the site preference gives hints to establish the sublattice model in so-called CALPHAD approach, as well as the distribution of the atoms to carry out an ab initio calculation. In this contribution, based on the crystallographic information of the prototype, we established the general sublattice model to describe the site occupy behaviour of constituent atoms, and then solved the site occupying fraction by thermodynamics calculation with the support of ab initio calculations.We will present our recently study on the fine microstructure and mechanical properties of some selective alloy phases, include Ti2AlNb-based orthorhombic phase (O phase), multi-principal component alloy (so-called high entropy alloys).We calculated the site occupying fraction and then established supercells, so that the exact atom distribution based on it site fraction were arranged randomly, thus an exact ab initio calculations were fulfilled to get the elastic properties of complex alloy phases.We compared the supercell results with those obtained using virtual crystal approximation (VCA), special quasirandom structure (SQS) model and the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA).A comparison between the theoretical results and the available experimental data demonstrates that it is possible to describe the fundamental microstructure and properties quantitatively.