Abstract: Aiming at the on-line diagnosis and prognosis of composite structures, a method for structural delamination diagnosis and remaining useful life (RUL) prediction was proposed based on structural health monitoring (SHM) and the Bayesian theory. Within the Bayesian probabilistic framework, an exponential model was adopted to describe the prior progression of the fatigue delamination in the composite structure. Then, on-line SHM data were incorporated for diagnosing the delamination state, as well as parameters of the damage area progression model. The posterior estimations denoted the diagnosis result, based on which the progression of the delamination area in the future was predicted, giving the RUL of the current composite structure. The proposed method was validated on the simulated fatigue delamination growth in a stiffened composite structure through the finite element method. The result shows the accuracy of this method for on-line diagnosing the delamination damage, as well as predicting the RUL of the structure.