The phase relation of Li2O-FeO-P2O5 ternary system under the 95 % Ar + 5 % H-2 atmosphere has been systematically investigated by X-ray diffraction (XRD), and there exist 8 binary compounds, 4 ternary compounds, 2 two-phase regions, and 17 three-phase regions. No other new lithium ferrous phosphates can be existed in the Li2O-FeO-P2O5 ternary system under the 95 % Ar + 5 % H-2 atmosphere. In this system, the Li1+xFe1-xPO4 solid solution phase with the homogeneous range of -0.15 <= x <= 0.06 is determined. Their corresponding lattice parameters are obtained by the refinement results, and the results show that the lattice parameters (a, b, c, V) vary linearly with the increasing amount of excess Li-ion (x > 0) or with the increasing amount of excess Fe-ion (x < 0). The phase diagram determined in this paper can provide more information about the phase relation of Li2O-FeO-P2O5 ternary system and serve as a guide for the future investigation of lithium iron phosphates in this system.