In this paper, confined fluid interfacial tensions (IFTs) in nanopores and their influential factors are studied. First, a new generalized equation of state (EOS) considering the pore radius, intermolecular interactions, and wall effect is developed analytically for calculating the thermodynamic phase behaviour of confined pure and mixing fluids in nanopores. Second, the modified model based on the new EOS and coupled with the parachor model, which also takes account for the capillary pressure and shifts of critical properties, is applied to calculate the IFTs in nanopores at different conditions. Third, the following four important factors are specifically studied to evaluate their effects on the IFTs in nanopores: feed gas to liquid ratio (FGLR), temperature, pore radius, and wall-effect distance. The newly-developed model is found to be accurate for vapour-liquid equilibrium (VLE) and IFT calculations in bulk phase and nanopores by comparing with the measured and calculated data in the literature. The IFTs in bulk phase of the pure and mixing hydrocarbon (HC) systems are always higher than those in nanopores. At low pressures, the calculated IFTs in nanopores from the new model are higher than those from the previously modified EOS, whereas they become almost equivalent at high pressures. The calculated IFTs of the simple HC systems in nanopores keep constant at different FGLRs while they are decreased by reducing the FGLRs for a multicomponent mixing HC system. Moreover, at low pressures, the gaseous CO2-mixing HC IFTs in bulk phase and nanopores are inferred to be lowered by increasing the temperature while the liquid/supercritical CO2-mixing HC IFTs may be increased. The temperature effect on the IFTs are weakened in nanopores at most pressures except for some extremely high pressure cases. The IFTs in nanopores are decreased with the reduction of pore radius but keep constant at delta(p)/r(p) >= 1.0.