Nanoporous polymers have been industrially used in valuable products such as large-scale separator sheet of Liion batteries, hollow fiber membranes for medical usage, and high-performance membrane and adsorbent for gas and water purification. New development of fabrication methodology of nanoporous polymers is therefore a key challenge that will make it possible to launch new applications. Here we will describe our new fabrication methods of nanoporous polymers that rely on solid-liquid phase separation phenomena. One is flash-freezing nanocrystallization method that is driven by nanocrystallization of solvent molecules and the other is physical gel precursor methods utilizing crystallization of polymers. Deep understanding of phase separation and crystallization mechanism is a key on the fabrication methods. Using commercially-available polymers such as polystyrene and engineering plastics, these methods provide a membrane, a fiber, and a monolith of mesoporous polymers with nanopores (10−100 nm in diameter) and large specific surface areas (over 300 m²/g). The mesoporous polymers are useful as separation membranes and adsorbents for water purification and gas separation, thermal insulators, and optical materials.