Aprotic lithium-oxygen (Li-O2) batteries promise high energy, but the cycle life has been plagued by two major obstacles, the insulating products and highly reactive singlet oxygen (1O2), which cause higher overpotential and parasitic reactions, respectively. A solid-state catalyst is known to reduce overpotential; however, it is unclear whether it affects 1O2 generation. Herein, Co3O4 was employed as the representative catalyst in Li-O2 batteries, and 1O2 generation was investigated by ex-situ and operando electron paramagnetic resonance (EPR) spectroscopy. By comparing a carbon nanotube (CNT) cathode with a Co3O4/CNT cathode, we find that 1O2 generation in the charge process can be suppressed by the Co3O4 catalyst. After carefully studying the discharge products on the two electrodes and the corresponding decomposition processes, we conclude that a LiO2-like species is responsible for the 1O2 generation during the early charge stage. The Co3O4 catalyst reduces the amount of LiO2-like species in discharge products, and thus the 1O2 formation is suppressed.