Using high speed photo technique, the effect of the oxygen concentration on intake air of Stirling engine fueled with diesel and ethanol combustion flame was studied. The experimental results showed that as the oxygen concentration increased, the flame stability was improved and the combustion flame propagation speed was increased; as the oxygen concentration in intake air increased, the  combustion flame length decreased and the flame lightness increased;  the flame in the combustion chamber was brighter when fueled with diesel than fueled with ethanol. The oxygen concentration had an inconspicuous effect on combustion mode and flame cone angle when injected gas was used.