Many concerns have been raised about the mechanism of cathode reaction in molten carbonate fuel cell (MCFC). The chemical behavior of oxide species at cathode in molten carbonate is a key for understanding the process of cathode reactions. In this paper, the variety and role of the oxide species in both bulk and thin-film of basic molten carbonates were investigated by using a novel in-situ Raman spectroscopy. The results indicated that the dominant oxide species under basic conditions was peroxide ion, and it was possible to transform into the oxygen of crystalline lattice during the lithium-doped process. It was demonstrated that in-situ Raman spectroscopic technique was a promising tool to elucidate the mechanism of electrode reaction in molecular level in the MCFC condition.