The aim of this paper is to analyse fibre/matrix debond crack growth during high stress cyclic tension–tension loading of unidirectional composites. The debond crack evolution analysis is based on fracture mechanics concepts that mode II energy release rate calculations are performed analytically for long debonds, where crack growth is self-similar, and numerically for short debonds by finite element method in combination with virtual crack closure technique. From the calculation results simple expressions are derived for an arbitrary mechanical and thermal loading case. Finally, the obtained expressions are applied in Paris law for debond growth simulations in cyclic tension–tension loading.