The new poly(silyl ester) has been prepared by the polycondensation reaction of 1,5-dichloro-1,1,5,5-tetramethyl-3,3-diphenyl-trisi1oxane with di-tert-butyl fumarate by the elimination of tert-butyl chloride as a driving force. To investigate the self-crosslinking reaction of the unsaturated poly(silyl ester), poly(1,1,5,5-tetramethyl-3,3-diphenyltrisiloxane) was self- crosslinked in the presence of 2, 2'-azobis(isobutyronitri1e) (AIBN) as a radical initiator without solvent. After the self-crosslinking, the unsaturated poly(silyl ester), which was viscous liquids, turned into solid product. The characterization of the poly(silyl ester) and the self-crosslinked product included 1H-NMR spectroscopy, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Comparisons were made between the linear poly(silyl ester) and the self-crosslinked products. It was found that after crosslinking, the important resonance signal for ethenylene (C=C) of the poly(silyl ester) reduced, which show that the crosslinking reaction is carried out. The self-crosslinked product exist the structures of linear isomer and four-membered ring isomer. The glass-transition temperatures of the self-crosslinked poly(silyl ester) was higher than that of the uncrosslinked poly(silyl ester), and the thermal stability of the self-crosslinked poly(silyl ester) was better than that of uncrosslinked poly(silyl ester).