Gas injection is considered as an effective recovery process that has been widely used in the worldwide oil industry. There are limited pilot field projects conducted on enhanced-oil-recovery (EOR) process by gas injection in shale-oil reservoirs. Although numerous studies have been conducted on gas injection in tight gas or oil reservoirs, the main recovery mechanism in shale-oil reservoirs is not well-understood. Diffusion plays an important role in the oil-recovery process in fractured-shale reservoirs. Most of the current studies on diffusion are performed in such a way that the producing pressure is equal to the initial reservoir pressure or core pressure; thus, the convective displacement is eliminated or minimized. One of the challenges is to evaluate the role of diffusion in field-scale displacements in the presence of viscous flow. This paper discusses the role of diffusion in improving oil recovery in fractured shale-oil reservoirs. Hoteit and Firoozabadi (2009) investigated the diffusion effect on recovery performance in a fractured gas/condensate reservoir. Their simulation results showed that molecular diffusion has a significant effect on gas recovery if the reservoir pressure is below the minimum miscible pressure. Modelling of the diffusion effect on ultimate oil recovery in extensively fractured shale reservoirs is crucial to the development of these marginal shale-oil or -gas projects. Evaluation of the recovery contribution from diffusion will provide important insights into the recovery mechanisms in intensely fractured shale-gas/oil reservoirs. Currently, a majority of the diffusion models were developed on the basis of the singleporosity model that demands tremendous grid refinement in intensely fractured shale-oil reservoirs. The grid refinement is necessary for surrounding the fracture intersections, which makes the system become computationally expensive. In this paper, the matrix/matrix and matrix/fracture diffusion is coupled in a dualpermeability model to overcome the drawback of the single-porosity model. The simulation results demonstrate that the EOR by gas injection in the Eagle Ford shale-oil reservoir will benefit from matrix/matrix and matrix/fracture molecular diffusion.