In this paper, thermodynamic miscibility in nanoconfined spaces is quantified and evaluated. First, an analytical generalized equation of state (EOS) is developed by considering the effects of pore radius, molecule-molecule interaction, and molecule-wall interaction at nanometer scale, on the basis of which four extended cubic EOS are proposed. Second, the analytical formulations of the confined fluid free energy of mixing and solubility parameter at nanometer scale are developed thermodynamically. Third, the free energy of mixing and solubility parameter are calculated under different conditions, by means of which the conditions and characteristics of the fluid miscibility at nanometer scale are specifically studied. Finally, two important factors, the temperature and molecule-wall (m-w) interaction, are specifically studied to evaluate and compare their effects on miscibility. The fluid miscibility benefits from the reduction of the pore radius, while the m-w interaction is detrimental to the development of miscibility. Moreover, the molecular size of the single largest molecule in the mixture along with the wall-effect region radius is determined to be the bottom limit of the pore radius, above which fluid miscibility can be achieved and improved by reducing the pore radius. It is found that the solubility parameter is a better quantitative indicator of fluid miscibility; the calculated results from the extended van der Waals, Soave-Redlich-Kwong, and Peng-Robinson EOS are better than those from the extended Redlich-Kwong EOS. Furthermore, a more fluid miscible state is found to be achieved by reducing the temperature and wall-effect region radius. The extent of the effect of temperature on the fluid miscibility of different mixtures can be different. More specifically, the so-called lean gas (i.e., N-2, CH4, and CO2)-induced miscibility through the vaporizing process is found to be affected by the temperature to a larger extent in comparison with the rich gas (i.e., C2H6, C3H8, and i- and n-C4H10)-induced miscibility through the condensing vaporizing process.