Two series of (GeTe)0.85(Pb1-xSmxTe)0.15 (x =0, 0.08, 0.16, 0.2, 0.24 and 0.28) and Ge0.75Pb0.2Sm0.03Te1-ySey (y =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were prepared by melting, quenching spark plasma sintering (SPS) techniques to study the effects of Sm and Se substitution on the phases and the thermoelectric properties of the PbTe-GeTe alloys.The Sm-doped (GeTe)0.85(Pb1-xSmxTe)0.15 alloys are the composites containing GeTe-based, PbTe-based and SmTe1.8 phases.Sm doping refines the grains, leading to dispersive nanosized SmTe1.8 phase of about 20 nm and PbTe of about 100 nm in the spinodal microstructure formed by the GeTe-based and PbTe-based phases, which reduces the thermal conductivity significantly and thus leads to an enhancement in the figure of merit ZT of the materials.The sample (GeTe)0.85(Pb0.8Sm0.2Te)0.15 shows the low thermal conductivity of 1.37 W·m-1K-1 and maximum figure of merit ZT of 1.3 at 723 K.As the substitution of Se for Te in Sin-doped Ge0.75Pb0.2Sm0.03Te1-ySey alloys, the tiny phase Sm2Se3 appears while the SmTe1.8 phase disappears.Substitution of Se for Te promotes the solid solubility of Pb in GeTe-based phase, leading to a reduction of the PbTe-based phase and eventually the complete disappearance if Se content y achieves 0.5.The substantial amount of solute Pb and Se atoms in GeTe-based phase, together with the twinning microstructures of GeTe-based phase, and the nanosized Sm2Se3 phase further enhance phonon scattering, which further reduces the thermal conductivity and thus improves the thermoelectric property of the samples.The maximum ZT of 1.5 at 723 K was obtained in the alloy Ge0.75Pb0.20 Sm0.03Te0.5Se0.5, much higher than that of its Sm and Se-free Ge-Pb-Te alloy.