Sulfonated multiblock copolynaphthalimides (co-SPIs) with block length of 5-20 were prepared by a two-pot polymerization method from 1,4,5,8-naphthalenetetracarboxylic dianhydride, sulfonated diamines of 2,2'-bis(4-sulfophenoxy)benzidine (BSPOB) and 4,4'-bis(4-aminophenoxy)biphenyl-3,3'-disulfonic acid (BAPBDS) and nonsulfonated diamines. The scanning transmission electron microscopy showed that the BSPOB-based multiblock co-SPI membranes had well-defined microphase-separated structure where the hydrophilic and hydrophobic layer-like domains were oriented in the plane direction of membrane. On the other hand, the BAPBDS-based multiblock co-SPIs and all the random co-SPIs showed the homogeneous morphology. The water uptake, anisotropic membrane swelling, anisotropic proton conductivity and polymer electrolyte fuel cell (PEFC) performance were investigated for the multiblock and random co-SPIs. The results demonstrated strong effects of the membrane morphology on these properties. The BSPOB-based multiblock co-SPI membranes exhibited the larger through-plane swelling and the lower through-plane conductivity than the random ones, and as a result exhibited the lower PEFC performances. This study gives good understanding of the morphology-property relationship in novel block architectures' design. (C) 2009 Elsevier Ltd. All rights reserved.