The degradation mechanism for the low voltage cathodoluminescence of and phosphors in vacuum fluorescent displays has been investigated by mass spectrometer, field emission scanning electron microscope observation, Auger electron spectroscopy, and x‐ray photoelectron spectroscopy. It has been found that the decomposition and evaporation of phosphors take place during electron excitation. This causes the deterioration of cathode filaments as well as the decrease in luminous efficiency of the phosphors.