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We investigated the effect of the carbon species in the air (oxygen) electrode, electrolyte concentration, and humidity in the supplied O2 gas on the Li-O2 reactions by using the operando XRD analysis. Regarding carbon species, we found that the over-potentials in the galvanostatic discharge-charge process were suppressed when using the KB carbon in the air electrode. The results of operando XRD measurements revealed that the Li2O2 formed on the KB had the smaller crystalline or more amorphous like structures, which could be one reason for faster reaction kinetics of Li2O2 dissolution. The discharge-charge curves of the cells with different concentration of LiTFSI/(G4)n electrolyte showed the slight difference but less differences in the Li2O2 formation and dissolution behaviors. In addition to the nature of Li2O2 products, reaction of Li-salts would also have ineligible effects. We also found that the higher humidity in oxygen produced more the LiOH and promoted the Li2O2 dissolution, which indicate that the LiOH formation could affect the Li2O2 morphologies or surface chemistries. Our present results demonstrated that the operando XRD measurement are useful for analyzing the reaction mechanism of Li-O2 battery. [ABSTRACT FROM AUTHOR]