Based on the important discovery that single-atom transition metal Ta can catalyze the transformation of graphite into diamond under ordinary or low pressure, this work introduces atomic layer deposition (ALD) technology to deposit Ta single atoms on the surface of graphite sheets. The effects of ALD reactant type and reaction time on the graphite-to-diamond phase transformation were systematically investigated through annealing treatment. The results show that when oxygen plasma is used as the reactant, more nanoparticles are formed after annealing, and lattice fringes corresponding to the diamond (111) and (110) planes are observed, indicating the successful transformation of graphite into diamond under low-pressure conditions. Appropriately extending the oxygen plasma reaction time can reduce structural defects and increase the content of amorphous carbon or sp³ carbon, whereas an excessively long reaction time suppresses diamond nucleation, which is attributed to the change in the chemical state of Ta atoms. This study provides a new approach for the controllable deposition of Ta single atoms by ALD and offers a promising route for the low-pressure transformation of graphite into diamond.