A new class of vinyl-functionalized polysiloxane-block-polyimides were synthesized and characterized. The copolyimides were derived from 3,3 ＇,4,4＇-benzophenonetetracarboxylic dianhydride (BTDA)/bis[4-(3-aminophenoxy)-phenyl]sulfone (BAPSM) combined with two kinds of vinyl-containing diaminopolysiloxanes and imidized by a solution method. They were highly soluble in various organic solvents. The vinyl groups of the copolyimides were reacted with various hydrosilane compounds for crosslinking. The crosslinked products became insoluble in polar solvents and were analysed and characterized by thermomechanical analysis (t.m.a.), dynamic mechanical analysis (d.m.a.), and C-13 and Si-29 nuclear magnetic resonance (n.m.r.). The tensile modulus of the copolyimides having a polysiloxane composition of 50 wt% increased approximately twofold after the crosslinking reaction, while that of the copolyimides with compositions of 10-30 wt% was similar to that of the original copolyimides. Their thermal expansion coefficient below the glass transition temperature (T-g), alpha 1, was nearly the same as that of the original copolyimides, while the coefficient above T-g, alpha 2, decreased drastically compared with that of the original copolyimides.