Novel ternary stannides and a plumbide, Na2Mg3X2 (X = Sn, Pb) and Na4Mg4Sn3, were synthesized by heating the corresponding elements. The crystal structures were determined by single-crystal X-ray diffraction analysis, and the electrical conductivities and Seebeck coefficients were measured. The crystal structures of Na2Mg3X2 [orthorhombic, a = 7.3066(9) A, b = 14.4559(13) A, c = 6.6433(7)A for X = Sn, a = 7.4272(11), b = 14.770(3), c = 6.6852(11)A for X = Pb] are based on the Mg5Ga2-type structure (space group Ibam, Z = 4). Na4Mg4Sn3 crystallizes in an orthorhombic cell [a = 6.879(3)A, b = 7.154(2)A, c = 22.285(7) A, space group Fmmm, Z = 4] with layers of disordered Na atom arrangement with defects. The electrical conductivities measured for the polycrystalline sintered samples of Na2Mg3Sn2, Na4Mg4Sn3, and Na2Mg3Pb2 were 1.9 × 105Sm-1 at 300 K, 1.6 × 105Sm-1 at 307 K and 3.3 × 105Sm-1 at 300 K, respectively. The Seebeck coefficients (S) of Na2Mg3Sn2, Na4Mg4Sn3, and Na2Mg3Pb2 were +47 to +72, +29 to +67, and +10 to +24 μV K-1, respectively, and increased with increasing temperature of 300-600 K.