While research has explored surface treatment effects on adhesive joints made by synthetic composites or metals, there is a lack of research on biocomposite adhesive joints. This work aims to explore the effects of the surface roughness, induced by a sandblasting treatment, on the Mode-I-behaviour of adhesive joints made by flax/epoxy composites. This work focuses on the roughness induced by a variation of the sandlasting pressure. Roughness measurements were carried out on biocomposite adherents for three pressures (5, 6 and 7 bar). A finite element model (FEM) of a double cantilever beam (DCB) with a cohesive description (cohesive zone model, CZM) of the adhesive was proposed. In a macroscopic way, the roughness effect is represented by the change in the CZM parameters, accordingly, an identification procedure for different roughness level, was proposed. Comparisons between experimental Mode-I tests and FE simulations reveal a significant correlation between surface roughness and joint behaviour. A roughness mean depth R ~3 μm, induced by a sandblasting pressure of 5 bar, appear to maximize the bonding strength (+16.6% with respect to 7 bar pressure-induced roughness R ~4 μm). The Mode-I fracture energy decreases with the increasing of the roughness mean depth R (-33% between 5 and 7 bar conditions). These results provide critical insights for optimizing manufacturing processes of flax/epoxy composite adhesive joints in industries striving for sustainable solutions.