Carbon nanotubes (CNTs) are coated on the green ZnGa2O4: Mn2+ phosphor to improve its cathodoluminescent intensity. X-ray diffraction pattern shows a single phase ZnGa2O4, and scanning electron image and Raman spectrum confirm the CNTs in CNT-phosphor composites. As an increase of CNTs in the ZnGa2O4: Mn2+ phosphor, its photoluminescent intensity is gradually decreased, whereas its cathodoluminescent intensity is maximized at the optimal CNTs concentrations. It can be explained in terms of two competing effects: the increased conductivity resulting in the decrease of charging effect, and the decreased transparency from the black-color CNTs' absorption of lights. (c) 2007 The Electrochemical Society.