Because of the effect of nanoscale confinement, the phase behavior of fluids confined in nanopores differs significantly from that observed in a PVT cell. In this paper, the cubic Peng Robinson equation of state (EOS) is coupled with capillary pressure equation and adsorption theory to investigate and represent the phase equilibria of pure components and their mixtures in cylindrical nanopores. The shift of critical properties is also taken into account. Because of the effect of an adsorption film, an improved Young-Laplace equation is adopted to simulate capillarity instead of the conventional equation. For the adsorption behavior, the experimental data of the adsorbent of silicalite are used to represent the adsorption behavior of hydrocarbons in nanopores. Then a prediction process for the behavior of methane, n-butane, n-pentane, n-hexane and their mixtures are performed. Furthermore, the results are compared against the available experimental data to validate the accuracy of this scheme. An actual Eagle Ford oil is also used to examine the performance of our scheme. Results indicate that the presence of an adsorption film can further increase the vapor liquid equilibrium constant (K-value) and capillary pressure of the confined pure-component fluid, especially in the nanopores with a few nanometers. The smaller the nanopore radius, the higher the deviation between the actual K-value and the estimated value. The capillary pressure presents a bilinear relationship with the pore radius in a log log plot. For a binary mixture, it is observed the higher the difference between the two components, the stronger the nanopore confinement effects. For a multicomponent mixture and the real Eagle Ford oil, as the pore radius reduces, the bubble point pressure is depressed and the dew point pressure is increased. When the adsorption film is neglected, the bubble point pressure is overestimated, and the dew point pressure is underestimated. For the Eagle Ford oil, when the nanopore radius is higher than about 100 nm, the behavior approaches the bulk value and the influence of nanopore confinement can be neglected. The depression of bubble point pressure of an Eagle Ford oil reservoir well explains the behavior of a long-lasting flat producing gas/oil ratio (GOR). The phase behavior of tight oil plays an important role in reserve evaluation and development process of tight oil reservoirs. This study will shed some important insights on the phase behavior of tight oil in nanopores.