Abstract: The characterization of fatigue damage of composite materials was discussed.The degradation of effective modulus was used as a measurement of damage. Using continuous damage mechanics(CDM) theory, a power relationship is setup between the damage rate (d D /d n ) and cyclic numbers. This provides a well model for the life prediction of composite materials under cyclic loading. Experimental procedures were also performed in order to investigate the damage development during fatigue process.It was found that damage development shows three different stages. This makes it possible to describe fatigue damage theoretically.