Amphiphilic polycations, p(DMAPAA-co-MPC-co-SA)s, with various cation contents were designed as novel micelle-like nonviral gene carriers. The carriers led to a higher transgene expression in COS-1 cells three times as high as the case of the linear-type polycations. The enhanced transgene expressions were found to be related to the disassembly of plasmid DNA/carrier polyplexes. It can be concluded that a topological factor of the carriers greatly influences on the gene expression efficiency.