Abstract: Typical damage states in unnotched [0₂/±45₂/90₂]_s composite laminate were studied by experimental observation,and the stiffness reduction rate caused by damages was measured in the laminate.Two 3-1} mechanical models in which transverse crack and delamination with transverse crack developing are put forward,and the stiffness redumion of the laminate is calculated by the model, and is in agreement with the experiment.The results show that the transverse crack and delamination are the major damage modes of the laminate,and the damage development of delamination is a main and stable process,and markedly leads to the decrease in stiffness of the laminate.