Quaternary alloys Mg₂Sn_0.4Si_0.6-xGe_x (x=0, 0.02, 0.05, 0.08 0.1, and 0.2) were prepared using induction melting followed by hot-pressing. Relative densities of the sintered samples were over 97% of the theoretical values. Multiple phases were detected in the samples. It was found that the Seebeck coefficient was sensitive to the content of Mg₂Ge and a maximum value of about 350 μV-K^-1 was obtained. The introduction of Ge increases the electrical conductivity and the thermal conductivity simultaneously. The mechanism of this phenomenon was discussed. A maximum dimensionless figure of merit, ZT, of about 0.28 was obtained for Mg₂Sn_0.4Si_0.55Ge_0.05 at 550 K.