The complex study of the thermoelectric properties in solid solutions between compounds Mg2X (X=Si, Ge, Sn) was accomplished. Analysis of the features of band structure, thermal conductivity and electrical properties in the wide range of temperature and carrier concentration has shown that the most effective thermoelectric can be achieved in the Mg2Si-Mg2Sn solid solution. Energy spectrum and carrier concentration optimization, and, also, lattice thermal conductivity minimization allowed to establish the most effective compounds for the thermoelectrics of n- and p- type. Thermoelectrics with the maximum dimensionless figure of merit of more than 1.2 and average ZT0.9 (in the temperature range 300-800K) were obtained with developed synthesis and doping techniques. These materials are cheap, wide spread and environment friendly, have non-toxic initial components. It is very favorable for practical application.