Fluorine-containing polyimide with crosslinkable vinyl group (FPI) was synthesized from 4,4 0 -(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,2 0 -bis(trifluoromethyl)-4,4 0 -diaminobiphenyl (PFMB), and 4-amino styrene (AS). The reinforced composite membranes based on semi-interpenetrating polymer networks (semi-IPN) were prepared via solution casting of FPI and Nafion Ò 212, and crosslinking thereafter. The water uptake, swelling ratio, mechanical properties, thermal behavior, proton conductivity, and oxidative stability of the composite membranes were investigated. Compared with the recast Nafion Ò 212, the composite membrane shows better mechanical properties and improved dimensional stability. The tensile strength of the composite membranes ranges from 39.0 MPa to 80.0 MPa, which is higher than that of the recast Nafion Ò 212 membrane (26.6 MPa). The dimensional stability of the composite membranes increases with increasing FPI content in the membranes, whereas the proton conductivity decreases. The composite membranes show considerable proton conductivity from 2.0 Â 10 À2 S cm À1 to 8.9 Â 10 À2 S cm À1 at a temperature from 30 C to 100 C, depending on the FPI contents. The composite membranes with semi-IPN from FPI and Nafion Ò 212 have considerable high proton conductivity, excellent mechanical properties, thermal and dimensional stabilities.