Mg2Si0.487-2xSn0.5(GaSb)xSb0.013 (0.04 ≤ x ≤ 0.10) solid solutions were synthesized by a B2O3 flux method followed by hot pressing. X-ray power diffraction analysis confirms that single-phased samples are obtained. It is found that the Sb-doping effectively enhances the electrical conductivity. The Seebeck coefficients increase while the electrical conductivity decreases for Mg2Si0.487-2xSn0.5(GaSb)xSb0.013 with the increase of temperature. With increasing of GaSb content the electrical conductivity first increases and then decreases. Among all the samples, Mg2Si0.287Sn0.5(GaSb)0.1Sb0.013 sample has the lowest lattice thermal conductivity which is about 15% lower than that of Mg2Si0.5Sn0.5 [11] at room temperature. A maximum dimensionless figure of merit of 0.61 at 720 K has been obtained for Mg2Si0.327Sn0.5(GaSb)0.08Sb0.013 mainly due to its high electrical conductivity, which is obviously higher than that (0.019 at 540 K) of Mg2Si0.5Sn0.5 [11]