The mechanism of degradation on the low voltage cathodoluminescence (CL) efficiency of SrTiO3 :Pr,Al,Ga phosphors is investigated. The effect of carbon overlayer adsorbed on the phosphor surface from the vacuum ambient during the electron irradiation done in a high vacuum chamber causes a rapid degradation of low voltage CL noticeably. Especially, based on Auger electron spectroscopy combined with CL spectroscopy it is observed that there is obvious correlation between carbon buildup and loss of CL brightness of the phosphors characterized in this study. Also, it is shown that the increased amount of carbon adsorbed during the electron irradiation on the SrTiO3 :Pr, Al, Ga phosphor surface is completely removed by heating the degraded SrTiO3 :Pr, Al, Ga samples at 470degreesC for 30 min in air. Furthermore, the decreased CL intensity of degraded samples is recovered up to that of original ones together with the removal of carbon. From the present work, it is understood that the rapid degradation of low voltage CL of the SrTiO3 :Pr, Al, Ga phosphors is directly related to the formation of the carbon overlayer, but not related to a change in the emission properties of the luminescent center themselves.