Photovoltaic(PV) module is generally consisted of series connected solar cells to adjust voltage. The performance of PV module installed in outdoor is affected by shading caused by various field environmental factors. Some bypass diodes are installed in PV module to prevent it from hotspot and mismatch power loss under partially shading condition. Typically, schottky diodes are used as bypass diode for rapid motion by reverse voltage. However, schottky diodes are very susceptible to static voltage discharge and mechanical stress. The damage causes of bypass diode is mostly attributed to high surge voltage, thermal runaway and initial defect. In this paper, a study on characteristic variation and degradation of PV module by damaged bypass diode is conducted. The electrical & thermal characteristics variation of damaged bypass diode is investigated under reverse bias. The damaged bypass diode changes is property to a conductor with a low resistance as reverse breakdown voltage. By measuring surface temperature of damaged bypass diode depending on the current, we found that the surface temperature of damaged bypass diode sharply rises from 4[A]. Moreover, the maximum surface temperature of damage bypass diode is about 181[℃] at 8[A](=Short circuit current of PV module). The electrical & thermal characteristics variation of PV module implementing damaged bypass diode is confirmed. Solar cells connected with damaged bypass diode in PV module is short by damaged bypass diode. So, damaged bypass diode make a closed circuit in PV module. To estimate power of PV module by damaged bypass diode, the equation is derived using solar cell equivalent circuit. The derived equation was simulated by Matlab tools, comparing measured data of PV module with damaged bypass diodes. The error between simulation value and measured value is about 0.1-0.2[V] at open voltage. But, the form of I-V curve and P-V curve between simulation and measurement is a close correspondence. The output of PV module with damaged bypass diode is proportionally reduced by the total number of connected solar cells per bypass diode. For this reason, temperature of solar cells connected with bypass diodes in PV module more rise about 20℃ than with normal bypass diode. Finally, empirical lifetime measurement of PV modules with normal bypass diode and damaged bypass diode during 8 month reveals that the degradation of PV module by damaged bypass diode rarely occur when an inverter operates. But, PV module with damaged bypass diode has a 2.67[W] reduction in output compared to the without damaged bypass diode during 4 month when the inverter not operate. When the inverter does not operate, it is confirmed that damaged bypass diode can be a degradation factor of PV module because experimental period is shorter than that of the inverter, but output decrease is about 6 times. As a result, output of PV module with damaged bypass can be estimated and bypass diode failure of PV module can be clearly determined using temperature characteristic of PV module with damaged bypass diode or damaged bypass diode and normal bypass diode.