The starting current of a simple model of the backward‐wave oscillator described by Kompfner and Williams has been calculated. The effect of space charge is included. The starting currentI0may be written in the formI0=4V0Z0a0(4QC)2&pgr;N3,whereV0is the beam voltage,Z0is the impedance of the circuit,Nis the length of the oscillator in wave‐lengths measured on the circuit anda0(4QC) is a dimensionless quantity which has been evaluated as a function of the space‐charge parameter 4QC.