In this paper, the effects of adhesion properties of anisotropic conductive films (ACFs) interconnection on the chip-in-flex (CIF) bending reliability were investigated. Oxygen plasma treatment was conducted to increase the adhesion strength between ACFs and Si chip or ACFs and flexible printed circuit (FPC) substrates. In order to characterize the enhanced adhesion properties of the CIF packages, surface energy, surface roughness, elemental composition, and peel strength were measured. A digital image correlation method was used with cross-sectional scanning electron microscopy images to visualize the stress development at the ACFs interconnection. It was found that the interface of ACFs resin and FPC substrate showed the weakest adhesion, where the delamination was initiated. As a result of the improved adhesion at the ACFs resin and FPC substrate, the location of stress concentration was changed to the interface of Si chip and ACFs resin, leading to better dynamic bending reliability. When the oxygen plasma was treated both on the Si chip and FPC substrate, the stress concentration was observed not at the ACFs interfaces, but inside of the ACFs resin, resulting in further improved dynamic bending reliability. With the optimized plasma treatment condition and the ACFs materials, the dynamic bending reliability of the CIF packages was successfully demonstrated up to 160 000 bending cycles at a 7.5-mm bending radius without any electrical failures.