The atomic structure of single crystal specimens of Fe-Ga alloys containing 4, 9, and 18 at % gallium has been studied by the X-ray diffraction method. The specimens were heat treated via two modes: annealing in paramagnetic state with subsequent quenching in water and annealing in ferromagnetic state with slow cooling to room temperature. Analysis of X-ray diffraction patterns indicates that the short-range order of the D0(3) type is formed in the alloy with 18 at % gallium; moreover the volume fraction of regions of the D0(3) phase considerably increases upon annealing. In the alloys with 4 and 9 at % gallium, the short-range order of the D0(3) type is absent. For all compositions, near the nodes (001), (003), and (111) we observe diffuse peaks whose intensities are independent of the heat treatment mode. It is shown that their appearance is related to the presence of small clusters of B2 type. The role of the observed structural peculiarities in the formation of magnetoelastic properties of Fe-Ga alloys is discussed.