The degradation behaviors of flip-chip 260 nm ultraviolet light emitting diodes (UVC-LEDs) were studied using in-situ accelerated system and different analytical technologies. The optical power of LEDs stressed at a constant DC current of 20 mA decreased to similar to 63% of the initial value after 150 h of operation. The failure mechanisms were investigated systematically by using current-voltage measurements, Secondary Ion Mass Spectroscopy, Transient Thermal Analysis, Scan Electron Microscopy, etc. The results show that: the concentration of H in p-GaN layer decreased from 4.5e17 atoms/cm(3) for the unstressed samples to 6.0e16 atoms/cm(3) for stressed sample, while there is no change for the Mg concentration, indicating the hydrogen dissociated from the Mg-H complex resulting from the activation of the Mg-dopant during the initial period; the increase of the current for the reverse bias region and the low-forward bias region during stress is due to the increase of defect-assisted carrier tunneling; the electro-migration of contact metal may generate the current leakage channel, which degraded the optical power of the LEDs. These results will help to improve the reliability design of AlGaN-based deep-UV LEDs.