This paper focuses on the effect of delamination on electrical performance in PV module, especially electrical loss of solar cell. Degradation mode of field aged crystalline-Si PV module has been investigated through visual microscope. Electrical performance was measured with solar simulator. Electroluminescence analysis was carried out for detecting defects of PV module. Delamination observed in the PV module has occurred at the interface between the encapsulant and the front surface of the solar cell and between the encapsulant and glass. Delamination was a main degradation mode of PV module. To describe the effect of delamination on the electrical performance, new analysis through line irradiance system for electrical mapping was used. Investigation of delamination indicated that new analysis is useful to describe the effect of delamination compared to the electroluminescence analysis.