In this work, polydimethylsiloxane (PDMS) with different molecular weight were used to modify diglycidyl ether of bisphenol A (DGEBA). A PDMS-block-DGEBA. copolymer that acted as a compatibilizer was prepared via DGEBA, hydroxy terminated PDMS, and silane coupling agent in the presence of a catalyst. The reaction mechanisms and compatibilizing effects of the block copolymer were studied by means of Fourier transform infrared (FTIR) spectrum analysis and observation under scanning electron microscopy (SEM). The thermal and mechanical behaviors were analysed as well. Results indicated that the block copolymer has good compatibilizing effect, and PDMS with low molecular weight could be dispersed in the DGEBA matrix more evenly. Moreover, the PDMS modified DGEBA systems had higher impact strength and lower weight loss rate than those of the pristine DGEBA system. (C) 2000 John Wiley & Sons, Inc.