concretely aiming at the efficient millimeter-wave power generation for monolithically integrated design, the small-signal S-parameter characterization of the impact-ionization avalanche transit-time (IMPATT) diodes was presented in this work. By combining the measured S-parameter data both of the 40 × 2 μm<sup>2</sup>IMPATT diode and different short ended coplanar waveguide (CPW) structures, a designing-by-characterizing procedure was demonstrated for a monolithic E-band IMPATT oscillator. In spite of much less tuning freedom compared with the conventional discrete design, the Kurokawa condition for steady oscillation using IMPATT diode as active device was still well caught and comprehensively demonstrated by all characterized data. According to measured amplification spectra, the oscillation condition was optimally met at 70.54 GHz under the biasing condition of 37.11 mA for the finalized oscillator, which agrees well with the design prediction. Without any extra cooling element, which is a must for conventional discrete design, the monolithic IMPATT oscillator (~ 0.2 mm<sup>2</sup>) was functional free of damage over 20 minutes under continuous-wave (CW) biasing modus.