La₂Mo₂O₉ is expected to have an extremely low thermal conductivity. However, the α-β phase transition at 580 oC seriously affects its performance and applications. In this work, the effect of Gd^₃₊ doping on the phase stability and thermal conductivity was investigated. The results show that the phase transition of La₂Mo₂O₉ is effectively suppressed with the doping of Gd^₃₊ ions. The samples show a β-phase when x ≥ 0.2. Moreover, the thermal conductivity decreases firstly and then increases with the increase of doping content of Gd^₃₊ ions with a minimum value at x = 0.2 at room temperature. The thermal conductivities of all samples are below 1 W/(m.K).