Abstract: Some of the mechanical behaviours of hybrid fibre composites were considered as the hybrid effect which is defined as a positive or negative deviation of a certain mechanical prorerty from the rule of mixture.The current theories about the hybrid effect were reviewed.The main factors affecting the hybrid effect were the properties of components,structure of hybrids and the interface between fibres and resin.The hybrid effect on the fracture strain of fibre and the strength of hybrids can be explaineds by the theories of fracture mechanics(thermodynamics) and micromechanics(statistics), The hybrid effect on fracture energy of hybrids depends on the hybrid structure and loading conditions.