The phase diagram of pseudo-binary system LiNaS04-MgSO4 system has been studied by means of DTA and X-ray diffraction technique. Three compounds (LiNa)0.8Mg0.20.8·Mg0.2 SO4 (LiNa)0.67Mg0.33 SO4 and (LiNa)0.4 Mg0.6 SO4 have been found in this system.The ionic conductions in these three compounds have been investigated. The ionic conductivities at 490℃ are 6.1×10-5S ·cm-1, 6.5×10-5S·cm-1 and 9.8×10-6S·cm-1 with activa tion energies 1.67 eV, 1.48 eV and 1.32 eV, respectively. The prototype high temperature thermal batteries Mg/MnO2 using these compounds respectively as electrolyte have been made and tested. The results show that, Mg/ (LiNa)0.8Mg0.2 SO4/MnO2 cell has better Polarization and discharg behaviour than the others. Hence (LiNa)0.8Mg0.2SO4 might be used as electrolyte in high temperature thermal battery.