The effects of heat treatment, microstructure and surface condition on thermal stability of the Ti811 alloy have been studied The Ti811 alloy with an equiaxed and bimode structure has good thermal stability When the structure is acicular, themal stability of the Ti811 alloy deteriorates obviously The surface oxide layer formed for exposure at 425℃ seems to have no effect on thermal stability The main factor influencing thermal stability is the variation of phase structure during heat treatment