2-ethylhexnoic acid was prepared by catalytic oxidation of 2-ethylhexanol using O2 as an oxidant under base-free condition. Quantitative analysis was performed by gas chromatography (GC), and the qualitative analysis was carried out by GC-MS. The effects of catalyst, solvent, reaction temperature, time and oxygen pressure on the reaction were investigated. The results showed that TEMPO/Cu(NO3)2?3H2O was the efficient catalyst system, and ethyl acetate was the suitable solvent. Under the optimum conditions , the yield of 2-ethylhexnoic acid could reach 77.1%, when TEMPO was 5% mole of 2-ethylhexanol, Cu(NO3)2?3H2O was 4% mole of 2-ethylhexanol, and reaction at 0.5MPa O2, 100℃ for 6h. Under the constant oxygen pressure, the reaction is a pseudo first order reaction and the activation energy is 59.9kJ/mol. The main by-products are 3-heptanol and 3-heptanone.