Abstract: An evaluation method for testing the defoaming performance of defoamers at high temperature was introduced, and it was used to compare defoamers for delayed coking process. The thermal stability of defoamers was investigated by TGA. Test results showed that the silicon-free defoamers were very unstable at high temperature, they decomposed at about 150℃ and lost defoaming function, thus they were not suitable for delayed coking process. Defoamers having organic silicon maintained good defoaming performance at temperature as high as 400℃, it was suggested that the proper dosage of these defoamers for delayed coking should be in the range of 20—50 ?g/g. Moreover, defoamer with crosslinking structure of polysiloxane exhibited good defoaming effect even at low dosage.