Fatigue tests were conducted on adhesively-bonded CFRP/aluminum double-cantilever beam (DCB) specimens to investigate the effect of adherend thickness on the fatigue crack growth rate. Components of the specimens were unidirectional composite, aluminum plate and a film-type epoxy adhesive. The experimental results indicate that the increase in the aluminum plate thickness lowers the fatigue threshold value and steepens the slope in the Paris region. To elucidate the effect of the adherend thickness on the fatigue crack propagation behavior, a finite element analysis was conducted to investigate the mode ratio, and stress and strain distributions near the crack tip, where the coefficient of thermal expansion mismatch between the aluminum and the CFRP plates was taken into account. The effect of the adherend thickness on the crack propagation rate has been tentatively explained in terms of stress distributions near the crack tip and crack propagation path.