Gas injection is considered as an effective recovery process that has been widely used in the worldwide. There are limited pilot field projects conducted on EOR process by gas injection in shale oil reservoirs. Although a lot of studies have been made on gas injection in tight gas or oil reservoirs, it is not clear to the public what the main recovery mechanism is in shale oil reservoirs. Diffusion is considered as an important factor in the oil recovery process in fractured shale reservoirs. Most of the current studies on diffusion are performed in a way that fixes the producing pressure equal to the initial reservoir or core pressure. In order to study the diffusion effect, the convective displacement is eliminated or minimized. In this paper, the role of diffusion on improving oil recovery in fractured Eagle Ford shale oil reservoirs is discussed. 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. Modeling of the diffusion effect on ultimate oil recovery in extensively fractured shale reservoir is crucial to the development of these marginal shale oil/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 single-porosity model that demands tremendous grid refinement in intensely fractured shale oil reservoirs. The grid refinement is necessary surrounding the fracture intersections that makes the system become computationally expensive. In this paper, the matrix-matrix and matrix-fracture diffusion is coupled into a dual permeability model to overcome the drawback of single-porosity model. The simulation results indicate that the stimulated natural fractures are critical to enhancing oil recovery and well productivity in shale oil reservoirs. Including diffusion effect in gas injection in fractured shale oil reservoirs achieves higher oil production. The simulation results demonstrate that the enhanced oil recovery by gas injection scheme in the Eagle Ford shale oil reservoir will benefit from matrix-matrix and matrix-fracture molecular diffusion, varying with the injection rate, intensity of natural fractures and other factors.