A muVT-ensemble Orientational-Bias Monte Carlo technique has been utilized to simulate adsorption isotherms of pure and mixed propane/propylene in NaX zeolite at 303 K. Propylene is adsorbed more strongly than propane, and the adsorption energy of propylene is found to be about 5 kJ/mol larger than that of propane mainly through the Coulomb interactions between molecules and zeolite atonns/Na ions. The adsorption isotherms of pure gases are correlated with four theoretical models: Langmuir, multisite occupancy, two-dimensional van der Waals fluid and Ruthven+U models. The latter three models are found to fit much better than the Langmuir model. The simulation isotherms of propane/propylene mixtures, calculated at 100 kPa in total pressure, are qualitatively well predicted by the latter three models.