A triode structure of field emission displays based upon carbon-nanotube emitters is studied. In this structure, gate electrodes are located underneath cathode electrodes with an in-between insulating layer, so called an under-gate-type triode structure. The modulation of electron emission by changing gate voltages is confirmed. Charging on the insulating layer by emission electrons could be reduced by pattering the insulating layer. Results of our experiment show that emission properties are improved in the triode structure with the patterned insulator. The simple structure and fabrication process may lead the under-gate-type triode structure to the practical applications. (C) 2003 Published by Elsevier B.V.