The effect of the cooling rate on in-plane and interlaminar properties of carbon fiber/semicrystalline PEEK matrix composites was studied. Strengths and moduli were measured in tension, flexure, and interlaminar shear, all of which were shown to correlate, to different degrees, with the fiber-matrix interface adhesion and the bulk matrix properties, The in-plane and interlaminar properties, in general, increased with a decreasing cooling rate, which was attributed to changes in the failure mechanism from adhesive failure involving fiber-matrix interface debonding at high cooling rates to matrix-dominant cohesive failure at low cooling rates. The present study demonstrates that the mechanical properties of semicrystalline thermoplastic composites can be tailored for desired applications by controlling the processing conditions, especially the cooling rate.