A study has been made of the effect of cooling rate on interlaminar fracture toughness of unidirectional carbon fibre–polyetheretherketone (PEEK) matrix composites. It is shown that the propagation values of both mode I and II propagation interlaminar fracture toughness increased with increasing cooling rate towards a saturation level for the range of cooling rate studied. The cooling rate dependency of the composite interlaminar fracture toughness is the result of complex interactions between two important properties, namely the matrix ductility and fibre–matrix interface bond strength. These two properties varied totally in an opposite manner against cooling rate through its effect on crystallinity: matrix ductility varied directly proportional to cooling rate while the converse is true for interface bond strength. The extent of plastic deformation of PEEK matrix contributed a predominant part to composite toughness, while an adequate interface bond is required to allow matrix deformation to take place to a full degree. A practical implication is that these two properties need to be optimised using an appropriate cooling rate to produce composites possessing high interlaminar fracture resistance.