The paper reports on an experimental study of the Mode I interlaminar fracture of unidirectional carbon fibers/epoxy resin composites. Mode I delamination strain energy release rate GIC was determined in double cantilever beam (DCB) test, before and after gamma irradiation at various doses. Glass transition temperature, Tg of epoxy matrix was determined from dynamic mechanical measurements. The delamination surfaces of tested coupons were observed by scanning electron microscopy. The variations in GIC values were correlated with irradiation doses, Tg values and the features of delamination microfractographs, as well as with the variation under irradiation of matrix or fibre/matrix dominated mechanical properties.