The effect of mean load on the fatigue behavior of adhesively-bonded pultruded GFRP double-lap joints under constant amplitude loading was experimentally investigated. The joints were examined under nine different stress ratios (R) representing pure tension, compression, and combined tension-compression fatigue loading. A transition of the failure mode, from tensile to compressive, was observed as the mean load decreased from positive to negative values. The slope of the S-N curves derived for R-ratios with positive or negative mean load consistently decreased with increasing mean load. The highest load amplitude corresponded to the R-ratio where the transition of the fatigue failure mode occurred (R = -2). The characteristics of the constant life diagram developed for the examined bonded joints are thoroughly discussed. A phenomenological formulation was proposed and its accuracy evaluated by comparisons with the derived experimental data. The comparison of the new formulation with models commonly used for composite materials proved its higher accuracy that is achieved with less implementation effort. © 2012 Elsevier Ltd. All rights reserved.