Abstract: A batch of monocrystalline modules that have been in service for 20 years in China, were selected to study the power attenuation at the end of their service life. For 132 pieces of monocrystalline module SM55 produced by Siemens Solar in 1996, EL and power related tests were carried out. The average power attenuation was 26.7% and annual average attenuation rate is 1.54%. A simple linear fitting function was used to fit the attenuation of the power with the attenuation of the current and the attenuation of the fill factor. It was found that the power reduction was mainly due to the reduction of the fill factor. Study and analyze the distribution of cracks and fragments in the component cell, and find that cracks and fragments were not the main factors causing component power attenuation. Combined with the environmental conditions in which the components were previously serviced, it was analyzed that the component power attenuation was mainly due to the increase in series resistance.