Abstract: A large area diode(12cm × 75cm) using a single insulating structure withoutgrading has been used in the research work of hundred-joule KrF laser.The distribution of elec-tric field lines and equipotential lines in REB(relativistic electron beam) diode has been calcu-lated and compared with the experimental results.There is no flashover occurred when the elec,tric field is less than 3kV/cm at the triple point(junction of a lucite wall with metallic wall anda vacuum).