PU(HTBN)／PMMA interpenetrating polymer networks (IPN’S) were synthesized by simultaneous polymerization，and were investigated by dynamic mechanical spectros-copy and transmission electron microscopy．Theoretical caleulation by Davie’s equation and experimental results showed that there are two continuous phases Which separate from each other，and two glass transition temperatures (Tg1，Tgl)corresponding to the components in the polymer networks．Owing to the“forced compatibility”produced by interpenetration．the phase domains are much finer in full．IPN’s (1000—2000 A) than in grafts (3000一6000 A) and semi-IPN，s(about 3000 A)，and become more smaller and uniform with the increase of phase continuity by appropriate crosslinkage．The reaction between some HTBN molecules and MMA bring about the increase of Tg1