The paper illustrates the effect of z-pins on the strain energy release rate in composite co-cured double cantilever beams (DCB) subject to a standard fracture toughness test. The conclusions obtained as a result of the solution illustrate that z-pins can provide drastic enhancement in fatigue and fracture properties of a co-cured z-pinned composite joint, even if their volume fraction is low. The strain energy release rate for loading and geometry combinations that do not result in immediate fracture was significantly reduced as a result of using z-pins. This slows the rate of the crack propagation if it is governed by the Paris law. Moreover, z-pins can completely arrest the crack. Although the analysis was performed for DCB specimens, the conclusions can be extrapolated to a general case of co-cured z-pinned joints.