The synthesis of thermoelectric materials (beta " -alumina) for the alkali metal thermoelectric converter (AMTEC) has been studied using a newly designed plasma jet reactor. Under not only atmospheric pressure but also low pressure, thin films of beta " -alumina are successfully synthesized for the first time from powder mixtures of alpha -Al2O3, Na2CO3 and MgO. The powder-mixing ratio, jet power and substrate position strongly affect the synthesis of beta " -alumina, which is found to be correlated with jet temperature. Synthesis of ferrite particles is also discussed. Under both atmospheric and lower pressures, ferrite particles are successfully synthesized from powder mixtures of Fe2O3, ZnO, NiO and CuO. In the high temperature region of the plasma jet, ferrite particles with high purity can be synthesized.