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Within the article, a preeroded copper cathode was subjected to the impact of a series of high-voltage pulses with a duration of < 1 ns for surface conditioning. An effective cathode surface area was ~0.01 mm2. Preerosion of the cathode surface was performed using a series of vacuum arc discharges of microsecond duration. As a result, a well-pronounced microrelief with a typical dimension of ~5– $10~\mu \text{m}$ was formed at the cathode surface. Then, to smooth the microrelief, the cathode was placed into a vacuum setup based on a RADAN pulse generator with a pulse duration of < 1 ns and a pulse amplitude of up to −140 kV. About 900 discharges of a subnanosecond duration were initiated. To estimate the conditioning effectiveness, the state of the cathode surface was controlled by studying the cathode field-emission characteristics and cathode surface imaging by scanning electron microscopy (SEM). The field-emission characteristic study made it possible to estimate the electric field enhancement factor $\beta $ near the cathode surface before and after the conditioning. It was shown that the subnanosecond conditioning led to a decrease in $\beta $ by a factor of 6–8 from ~170 after eroding to ~20–30. Employing SEM investigation of the sample surface morphology, it was found that a typical microrelief dimension was decreased from ~5–10 to ~1– $2~\mu \text{m}$ or less. That is, a proposed conditioning technique allowed significant smoothing of the cathode surface.