The aerobic oxidation of cyclohexanol to cyclohexanone was conducted in compressed CO2 With a copper-based catalyst, and the effect of phase behavior and a small amount of cosolvents (toluene or fluorobenzene) on the conversion and selectivity of the reaction in supercritical (SC) CO2 was studied. To explore the advantages of the reaction in compressed CO2, the reaction was also performed in liquid toluene and fluorobenzene and in the absence of solvent for comparison. The results indicated that the selectivity of the reaction in compressed CO2 with and without cosolvents was much higher than that in the liquid solvents or without solvent. The conversion of cyclohexanol was improved effectively by adding a small amount of the cosolvents and the selectivity for the desired product was still very high. It was advantageous to carry out the reaction in CO2.