The structures of Ga atoms deposited on Ni(lll) surface was studied. When the coverage is less than one monolayer, two superstructures. (3(1/2)×3(1/2)) R30°and (2×2), have been observed, by means of LEED and AES.A quasi-equilibrium phase diagram has been drafted, and thus the rangs of coverage and temperature while these superstructures appear can be estimated.