As the first stage of development of a nanofocused x-ray source for high-resolution x-ray radiography, a commercial compact scanning electron microscope (SEM) was modified by replacing the thermionic cathode and two magnetic condenser lenses with a multiwalled carbon nanotube (MWCNT) bundle field emission (FE) cathode and a special designed Butler electrostatic lens, respectively. This modification also brought about a downsizing to the electron optical system, only 192 mm of distance between the cathode and the sample stage was achieved. The MWCNT bundle FE cathode continuously emitted electrons for over 9 h under a base pressure of ca. 10(-5) Torr which was a severer pressure for steady FE. The performance of electron gun system consisted of the MWCNT bundle FE cathode and the special Butler electrostatic lens was demonstrated by SEM images taken under the poor vacuum condition at room temperature. (c) 2007 American Vacuum Society.