Molecular diffusion effect has been ignored in many conventional reservoir studies, but it can play a significant role in tight fractured reservoirs and is crucial for an appropriate reservoir evaluation. We have characterized diffusion coefficients of methane gas in hydrocarbon fluid samples by combining experiment and simulation.We employ the theory of Fickian diffusive flux to evaluate molecular diffusion behavior. It is described as a product of concentration gradient, molar density and Fickian diffusion coefficient. Diffusion coefficients, although commonly estimated by existing correlations, shall be calibrated against actual molecular diffusion behavior for practical use. There have been few published papers showing Fickian multicomponent diffusion coefficients, because a simple and reliable measurement method has not been available. We hereby propose a method for computer-assisted multicomponent diffusion coefficient evaluation based on experiments proposed by Riazi that uses a PVT cell apparatus.We measure molecular diffusion behavior in a PVT cell and evaluate diffusion coefficients using Leahy-Dios and Firoozabadi's model with the assumption of local equilibrium described by the Peng-Robinson EOS. Volume shift parameters, binary interaction coefficients, initial pressure, initial liquid volume and diffusion coefficients are optimized to fit measurements by a new global optimization algorithm named iterative Latin hypercube samplings.Simulation case studies are performed to show the effect of molecular diffusion in tight fractured reservoirs. Results indicate that injected gas diffuses deeply into rock matrices when diffusive flux is properly represented. Consequently, better sweep efficiency is achieved compared to the cases without considering diffusion.In this paper, we present a systematic method to evaluate diffusion coefficients of reservoir fluids using EOS. It will be useful for reservoir simulation of oil and gas recovery in tight fractured reservoirs. 1 2 1 n J J J J  Reservoir Simulation Model. To demonstrate the effect of molecular diffusion in reservoirs, simulation case studies are performed. We use a fractured reservoir simulator for gas injection (FRSG) developed by Moortgat et al. (2009). It simulates 2-phase compositional flow in 2-dimentional (2D) fractured reservoirs accompanying Fickian diffusion with gravity and compositional effects. The program is based on mixed hybrid finite element (MHFE) and discontinuous Galerkin (DG) methods. Dij's are internally calculated by Leahy-Dios and Firoozabadi's correlation, similarly to our experimental approach. Also, it allows for adjusting Dij's by specifying multipliers M L and M G . Hence we can use Dij's which have been characterized through our experiments.We set-up a 2D cross-sectional (vertical) reservoir model with 12 fractures shown in Fig. 6. Permeability and porosity of rock matrices are 0.001 md and 0.012, respectively. Fracture permeability is given uniformly at 1.0  10 5 md. The model is designated to incorporate gravity drainag...