The results calculated by computer for the double-drift-region IMPATT diode on 3 mm waveband are reported. A comparison between single-drift and double-drift diodes is presented. The advantage of double-drift devices in respect to power output and efficiency is given. At the same time, effects of doping profile, current density and RF voltage on the performences of these devices are investigated. The theoretical data for design of double-drift IMPATT oscillator and amplifier on 8 mm waveband is also given.