Abstract: Heat is detrimental to the growth and flowering of Oncidium. Thus, a simulated high temperature stress in an incubator with light bulbs was imposed on Oncidium plants to examine the changes on the physiology of apical leaves. The physiological indices, including the contents of chlorophyll, soluble sugar, reducing sugar and proline (Pro) as well as the relative conductivity and peroxidase (POD) activity, were monitored. The results showed that, under the stress, (a) the relative conductivity and Pro content in the leaves significantly increased, while chlorophyll decreased; (b) the soluble sugar content became significantly higher than that of control in 21 d, but significantly lower in 42 d; (c) in comparison with control, the reducing sugar content was significantly higher in 7 d, but significantly lower after 35 d; and, (d) the POD activity increased with prolonged heat stress, it was lower during 7-21 d but significantly higher than that of control after 35 d. It appeared that the relative conductivity and the contents of Pro and chlorophyll in the apical leaves could be satisfactorily used as key indicators in evaluating the tolerance of an Oncidium plant to high temperatures.