Thermoelectric materials (beta"-alumina) for the alkali metal thermoelectric converter (AMTEC) have been synthesized with the use of a thermal plasma reactor based on the forced constricted type plasma jet generator. It is reconfirmed that the powder mixing ratio (i.e. the ratio between alpha-Al2O3, Na2CO3 and MgO), jet power and substrate position, strongly affect the synthesis of beta"-alumina. Besides these, the synthesis of beta"-alumina is also enhanced in the high-temperature region by oxygen injection. Those dependences are correlated with jet temperature and particle heating. The metal elect-rode of molybdenum is also prepared by spraying Mo powders. Porosity of the prepared films could be well controlled by varying the Pressure.