Abstract: The reliability of solar cells is the key factor to determine the performance and life of photovoltaic products. Many researchs have shown that the degradation and reliability of solar cells in practical applications are closely related to the defect properties of solar cells. In the paper,1/f noise is used to characterize the defect properties of amorphous silicon thin film solar cells. The results show that 1/f noise in amorphous silicon cells is caused by carrier mobility fluctuation caused by band-tail defects. The role of defects in the process of light-induced degradation（LID）is analyzed by noise model. The validity and sensitivity of noise in characterizing LID are verified by comparing with conventional electrical parameters.