Abstract: The output properties of electron beams extracted from different foilless diodes,with different voltages and different guiding magnetic fields, were investigatedby using a 2(1/2)dimensional computer code. The space charge effect, all componentsof the self-magnetic field of beam and three dimensional motions of electronswere considered. The Child-Langmuir emission law was employed and inhomo-geneous difference scheme was used in the simulation. The calculation resultshows that the diode geometry, applied voltage and guiding magnetic field havegreat effects on the characteristics of electron beam. The stronger the guidingmagnetic field is,the greater output beam current, with lower beam divergence,will be obtained. When the magnetic field exceeds a certain value, the beamcurrent no longer increases and decreases at a moderate level.