Flexible and transparent epoxy resin is fabricated by adding modified resin (MR) into diglycidyl ether of bisphenol A (DGEBA). The MR is prepared by the ring‐opening of the epoxy functional group of the DGEBA. The mechanical properties of the epoxy resin are tuned by adjusting the weight ratio of MR to DGEBA. When the MR content is 25 wt%, the elongation at break and the tensile strength of the epoxy resin can reach 69.1% and 27.0 MPa, respectively. Cured blends of DGEBA and MR are characterized using scanning electron microscopy, dynamic mechanical analysis and thermogravimetric analysis to reveal the mechanism of improving the flexibility of the epoxy resin while maintaining excellent transparency. © 2016 Society of Chemical Industry