Damage ahead of notch tips in continuous fiber-reinforced cross-ply composites consists of microcracks extendibg in the directions parallel to fibers in each ply. We direct our attention to the behaviour of these micrcracks in order to clarify the essential factor which drivdes microcracks, i. e., governs the extension behaviour of the damage region. We first determine quantitatively the state of damage with parameters which are local microcrack densities averaged over the thickness of the specimen, and we derive the relation between the increment of macroscopic material compliance and the elastic strain energy released by microcrack extension. With an assumption that the released energy would control the process of microcrack extension, a computer simulation of damage extension is carried out and its results appear to be in agreement with experimental results. We finally come to the conclusion that the damage extension in continuous fiber-reinforced cross-ply composites is essentially the release of elastic strain energy.