We have studied the phase separation and ordering phenomeon of InAlAs epilayers grown on InP substrate by LP-MOCVD with DCXRD, PL, and TEM. From the intensity and FWHM of DCXRD and PL, we observed that the structural and optical quality of InAlAs epilayers were improved as growth temperature increased. The band-gap reduction due to phase separation and ordering is 291, 246, and 28 meV in the InAlAs epilayers grown at $565^{circ}C$, $615^{circ}C$, and $700^{circ}C$, respectively, and shows the same from the InAlAs epilayer town at 5$65^{circ}C$ in which the HRTEM micrograph showed the lattice fringe between InAs-rich and AlAs-rich regions was tilted by $2^{circ}$ due to composition difference. However the maximum degree of ordering by intensity of extra spots was obtained at medium growth temperature. The annealing experiment by RTA of sample grown at $565^{circ}$ shows a maximum band-gap shift of 78eV at $880^{circ}$ for 3 min, and TEM shows that the origin of the blue shift of band-gap is the complete disappearance of ordering. Through annealing we can conclude that short time annealing affects only ordering and that most of the total band-gap reduction (~3/4) occurs by phase separation.