The fabrication of nanostructured thin films is important in a variety of applications. This article presents a fabrication method for introducing nanoscopic pores into polymeric thin films of block copolymers. As a model of this method, poly(styrene-b-2-vinylpyridine) (PS-PVP) thin film of which the original thickness is about 150 nm was employed with mixtures of methanol and CO2. Neither methanol nor CO2 alone can introduce nanopores into the PS-PVP thin films; however, the mixtures of methanol and CO2 can effectively localize methanol into PVP domains and leave nanopores after being removed. Methanol alone has a strong affinity for PVP blocks and potentially swells the PVP domains, but the glassy PS matrix prevents them from swelling. Therefore, it is essential for the successful introduction of nanopores to reduce the glass-transition temperature (T-g) Of PS by plasticizing with CO2. Compressed CO2, which reduces the T-g of PS, can quickly be removed from the mixture upon depressurization and increases the T-g of PS to prevent nanopores from collapsing. The size of the nanopores can be successfully adjusted by changing the pressure of CO2. On our experimental time scale, the morphologies were riot equilibrium, and the resultant nanoporous structures depend on the initial morphologies before the process.