The effect of an in situ Cs treatment on the field emission of a multi-walled carbon nanotube (MWNT) was investigated. The field emission of as-grown MWNT shows two Fowler-Nordheim (F-N) slopes in current-voltage characteristics, Both slopes in the two regions (high voltage and low voltage) were lowered as the result of the Cs treatment. The turn-on voltage was significantly decreased by a factor of about 1.3 and the total emission current was increased by over one order of magnitude after Cs deposition. In addition, the emission current was decreased beyond the optimum Cs deposition time. A stable emission current was observed after Cs deposition and during the no-Cs-deposition state. A decrease in work function is the major cause of the electron field enhancement as the result of the formation of a surface dipole layer induced by Cs deposition. (C) 2002 Elsevier Science B.V. All rights reserved.