•Bonding performance of epoxy coatings improved logistically as surface roughness increased.•Thinner coatings showed better bonding performance than thicker ones especially on deformability.•Cohesive failure could be only achieved on highly rough substrate with thinner coating thickness. The bonding performance between adherend and adhesive depends on two main factors including the surface roughness of the adherend and the bondline thickness of the adhesive. This paper investigated how the surface roughness of the adherend substrate and bondline thickness of the adhesive influence the bonding performance of epoxy adhesive joints on A36 mild steel substrates using single lap shear (SLS) tests. The evaluated bonding performances included the bonding strength and the maximum strain of epoxy adhesive joints which were fabricated with four different surface roughnesses and three different bondline thicknesses using a general-purpose epoxy adhesive. The experimental results indicated that rougher steel substrates or thinner epoxy adhesive bondlines yielded larger bonding strengths and higher maximum shear strains. In addition, the morphology of the fracture substrates before and after SLS tests were analyzed using scanning electron microscopy (SEM). The SEM image analyses indicated the improved bonding performance on very rough substrates was because of strong mechanical interlocking. Thinner epoxy adhesive bondlines had larger capabilities of plastic deformation as a result of enhanced toughness.