Disclosed is a method of in-situ monitoring a reduction process of uranium oxides by lithium metal, wherein a conversion yield of uranium metal from uranium oxides upon production of uranium metal through a reaction of uranium oxides (UO.sub.x, x.ltoreq.3) with lithium metal in the presence of a high-temperature molten salt is measured according to an electrochemical analysis based on an oxidation of an oxygen ion and a reduction of a lithium ion dissociated from lithium oxide obtained as a by-product of the reaction, by use of a measuring device composed of a potentiostat/galvanostat and a reactor provided with an anode and a cathode. The in-situ monitoring method of the current invention is advantageous in terms of fast and simplified measuring techniques, by directly measuring the reduction process of uranium oxides at the anode and cathode connected to the potentiostat/galvanostat in the presence of the high-temperature molten salt.