A series of polysiloxanes modified with fluoroalkyls and polyethers were synthesized. Antifoaming performance of such modified polysiloxanes in the mixture of diesel and mobile fuel was studied. The effects of factors such as polyethers type, molar ratio between polyether and fluoroalkyl, hydrogen-contents and viscosity of polysiloxanes, length of fluoroalkyls, etc., on the performance of modified products were discussed. It shows that polysiloxane modified with allyl polyoxypropylene ether and hexafluorobutyl acrylate has better breaking and inhibiting foam performance, when compared with that replacing allyl polyoxypropylene ether with allyl polyoxyethylene polyoxypropylene block ether and allyl polyoxyethylene ether. And when the molar ratio between allyl polyoxypropylene ether and hexafluorobutyl acrylate is 3 : 7, modified polysloxane has the best foam controlling performance. Low viscosity polysiloxane or low hydrogen content polysiloxane modified with allyl polyoxypropylene ether and hexafluorobutyl acrylate has better foam controlling performance. Polysiloxane modified with longer carbon chain fluoroalkyl and allyl polyoxypropylene ether has better antifoaming performance than that modified with shorter carbon chain fluoroalkyl and polyether and lower surface tension. The change of surface tension of foaming liquid between initially added with modified polysiloxane and after testing antifoaming performance was studied, and it shows that after testing performance of antifoam the surface tension of foaming liquid becomes bigger which is one possible reason for antifoam exhausting. (C) 2008 Wiley Periodicals, Inc.