Abstract: In this paper, the effects of the geometrical configuration prameters, including the cathode radius Rc, the anode –cathode gap dA-K, the anode aperture radius Ra and the cathode traverse length L, on the electron beams on foilless diode are studied using a numerical simulation method. The basic criterion of the selection of parameters such as dA-K≥3Rc, 1.7≤Ra/Rc≤1.8, and L/Rc=2 ~3 is obtained. In this condition, the high –brightness electron beam can be easily generated. The optimization of parameters of foilless diode model according to the realistic circumstances is given.