Delamination crack growth behavior under mode II cyclic loading was investigated with interlayer toughened CFRP laminate, T800H/3900-2. Fatigue tests under a constant maximum energy release rate showed that the growth rate was independent of the crack length. Then, load-shedding tests were carried out under various stress ratios. The fatigue crack growth resistance of the interlayer toughened CFRP was higher that of conventional CF/epoxy. The crack growth rate under various stress ratios was well correlated to the stress intensity range near the threshold region. Thus, the mechanism of fatigue fracture under mode II loading is completely different from that under mode I loading. Fractographic observation showed that the fatigue fracture occurred on the principal shear stress plane. Large hackle patterns which are typical for static mode II tests were not observed under fatigue loading. A mesoscopic fracture model was proposed to explain the microscopic observation and the stress ratio dependence.