The study of Mg2Si based thermoelectric materials has received widespread attention. In this research, quaternary Mg2(1+x)(Si0.2Ge0.1Sn0.7)(0.99)Sb-0.01 (0.06 <= x <= 0.12) solid solutions with an optimized Sb doping were prepared by B2O3 flux method combined with spark plasma sintering ( SPS) technique. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured as a function of Mg excess between 300 K and 780 K. The electron concentration, electrical conductivity and lattice thermal conductivity increase while the Seebeck coefficient decreases with increasing magnesium excess content. The electron effective mass enhancement for x >= 0.08 suggests the conduction band convergence of Mg2Si0.2Ge0.1Sn0.7. Mg-2.16(Si0.2Ge0.1Sn0.7)(0.99)Sb-0.01 with a maximum dimensionless figure of merit of 0.94 at 780 K stand out as one of the best materials for intermediate temperature applications, providing a good nontoxic alternative to PbTe. (C) 2015 Elsevier B.V. All rights reserved.