Polycrystalline diamond chemical vapor deposition (CVD) films have been etched in both microwave and hot filament direct current plasmas. Hydrogen, hydrogen/argon and hydrogen/oxygen mixtures were used as reactive gases. Our results showed that bias-induced electron bombardment in hydrogen plasma did not enhance the etching yield. In contrast, bias-induced ion bombardment in hydrogen promoted the increase of etching yield through the graphitization of diamond surface. Introduction of argon to the reactant precursors increased the ion bombardment, which in turn, led to higher defect density and the formation of non-diamond phase. The oxygen addition into hydrogen gas reverted the etching mechanism. in addition, oxygen acted as a medium for material transport in the case of hot filament plasma.