Based on the framework of systematic science of alloys, the basic information about states, volumes and potential energies of characteristic atoms in FCC Ti-Al system has been determined. For ordered FCC TiAl type alloys, the functions of the atomic states, atomic potential energies, cohesive energies, atomic volumes, the lattice constants and axis ratio c=a 0 varying with concentrations have also been established. The extreme points of curves vBx Al ; aBx Al and c=a 0 Bx Al are all at the x Al =70%. When :454 ; calculated lattice constants a 0 ΒΌ 0:39954 nm, c ΒΌ 0:40777 nm, c=a 0 ΒΌ 1:0206; which are in good agreement with experimental values. When x Ti =x Al > 1; c Ti 8 atoms decrease and c Ti 7 ; c Ti 6 y atoms appear in succession with the increase of x Ti ; c Al 4 atoms decrease and c Al 3 ; c Al 2 y atoms appear one after another. Because the influence of the change of Ti atomic states exceeds that of the change of Al atomic states, rich Ti-FCC TiAl alloys have larger atomic volumes, larger lattice constants, c=a 0 much closer to unity and comparatively good ductility, though the stability of FCC lattice is reduced. When x Ti =x Al o1; with the increase of x Al ; the atomic states c Ti 8 and c Al 4 changes in the direction opposite to the case with x Ti =x Al > 1: Thus, ordered rich Al-FCC TiAl alloys have smaller atomic volumes, smaller lattice constants, larger c=a 0 which depart further from unity and poorer ductility, and there is an increase of stability for the FCC lattice.