Block copolymers (BCPs) are interesting precursors to nanoporous films with well-defined and tunable porosities. So far, BCPs with polystyrene (PS) as the majority block are predominantly used, hindering the using of produced nanoporous films at elevated temperatures. Herein, we propose to use BCPs with poly (styrene-alt-N-phenylmaleimide) (P(S-alt-NMI)) as the majority block to develop thermally stable nanoporous films considering that this alternative copolymer has a much higher glass transition temperature than PS. We synthesized BCPs of poly (2-dimethylaminoethyl methacrylate) (PDMAEMA) and P(S-alt-NMI) (PDMAEMA-b-P(S-alt-NMI)) with four different block ratios by reversible addition-fragmentation chain transfer (RAFT) polymerization. The swelling behaviors of these BCPs in various solvents were investigated, and we found that nanoporosities will be generated in the BCP films following the mechanism of selective swelling induced pore generation when single solvents or solvent pairs having strong affinity to PDMAEMA and moderate affinity to P(S-alt-NMI) were used as the swelling agents. The pore sizes and porosities of thus-produced nanoporous films can be flexibly tuned by changing the compositions of the BCPs and the swelling conditions. Because of the excellent thermal stability of P(S-alt-NMI) blocks the nanoporous BCP films exhibited excellent thermal tolerance and they maintained intact at a temperature as high as 158 degrees C while the PS-based BCPs started to deform at the temperature of 75 degrees C. This thermal tolerance is very much desired in many applications, for example, high-temperature disinfection for medical purposes.