It is required a joint technic with excellent mechanical properties to fabricate glass fiber reinforced plastic (GFRP) components. However, it is well-known that the mechanical jointing often causes the fracture of fiber reinforced plastics. It seems that the adhesive bonding is suitable to joint GFRPs, and it is necessary to develop an adhesive with outstanding mechanical properties. Recently, cellulose nanofibers (CNFs) have been obtained from plants by mechanical and chemical methods and it has been reported that CNFs have remarkable mechanical properties. In this study, we inserted epoxy resin with CNFs between GFRP and investigated its bending behavior, to estimate the capability of epoxy resin with CNFs as an adhesive for GFRPs. The flexural behavior and modulus of GFRP was similar regardless of the inserted part of epoxy resin with CNFs layer in GFRP. The flexural strength of GFRP was drastically increased (up to 600 MPa) by the insert of epoxy resin with CNFs of 5 or 10 wt.%, although that of GFRP was decreased by the insert of epoxy resin with CNFs of 25 wt.%. Therefore, it seems that the epoxy resin with CNFs is available as an adhesive for GFRP. However, it is required the observation of CNFs dispersion in epoxy resin to understand the strengthening mechanism of epoxy resin with CNFs.