A new technique of contamination‐free thermochemical processing of diamond is proposed. A diamond plate, which is placed on a metal plate, is treated thermochemically from its lower surface, in liquid, using a laser as the heat source. A focused Q‐switched YAG laser beam was introduced, which passed through the liquid and diamond, onto the metal plate. The laser beam heats up the metal, and thereby, the lower surface of the diamond is etched by thermochemical interaction with the liquid. Comparison of the liquids, de‐ionized water, and KOH solution, proved that KOH solution is preferable to water with respect to the stability of machining and smoothness of the prepared surface.