The work function of a metal for electron source is one of the measures for evaluating the ability of electron emission, but the values are different depending on the crystal surfaces. In case of molybdenum, the Mo(100) surface has a relatively high value of 4.3 eV, however it can be reduced to 2.1 eV by heating with a slight layer of zirconium oxide. It is observed that field emission electrons are dominantly extracted from a (100) surface of a molybdenum sharp needle emitter, when the surface is modified with a zirconium oxide. The work function of the surface is estimated from Fowler-Nordheim plots. The estimated average value is 2.1 eV, which is remarkably small compared with a commonly accepted value of work function for a ZrO/W(100) thermal field emitter, namely 2.7-2.9 eV, while the smallest reported value is 2.4 eV. It is expected that smaller values of work function are desirable for cathode materials to get fine focused electron beams, because less chromatic aberration can be realized. It should be noticed from practical view points that the reduced work function is reproduced easily by applying several times of flash heating after being exposed to air. (c) 2005 American Vacuum Society.