The field electron emission properties of an individual. beta-rhombohedral boron nanowire (beta-r BNW) with electrode separation at nanoscale have been studied by ultrahigh vacuum four-probe scanning tunneling microscope (STM) system. A reproducible and stable emission current can be obtained. The maximal emission current density of individual boron nanowire is about 5 x 10(4) A/cm(2) at a low bias voltage (80V). An obvious deviation from the Fowler-Nordheim (FN) theory appears, when the electrode separation reduced below 120 nm. This deviation is tentatively assumed to due to the invalidation of free electron cloud approximation in FN theory. (C) 2011 Elsevier B.V. All rights reserved.