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The ultra-smooth surface and hydrophobic nitrogen-incorporated ultra-nano-crystalline diamond (N-UNCD) was directly synthesized by equilibrating the etching effect of OH radical and growth promotion of CN and CH in the 5% CH 4 added H 2 plasma environment with additional feeding of constant 0.16% O 2 and from 0.3% to 3.3% N 2 contents. The initially decreasing and then increasing trend of surface roughness, i.e., from as-grown appearance with pits to smooth and then to worm-like surface, was resulted from the offset and even suppressive effect on OH etched holes by the faster growth rate that under the help of more N 2 promoted chemical groups of CH and CN. In addition, chemical composition, i.e., the large amount of sp 2 phases (sp 2/ sp 3 ratio up to 1.399) and H termination of N-UNCD surface (proportion was 75.54 ± 3.8%), played an another enhancing function for repelling water (contact angle from 81.3° to 93.8°) although the surface roughness decreased to smoothest of 1.36 nm in Root-Mean-Square (RMS) in the presence of N 2 addition from 0.3% to 1%. However, continuing adding N 2 to 3.3%, the formed surface (RMS 8.98 nm) with worm-like ultra-nano diamond crystallites together with higher H reconstructed sp 2 contents (sp 2/ sp 3 ratio rose to 1.478) further boosted the hydrophobicity, at which the contact angle was finally increased to 110.2°. Therefore, in C-H-O-N gas system, ultra-smooth and uniform N-UNCD surface with excellent hydrophobicity can be obtained by a one-step method without any post-treatment. • Ultra-smooth and hydrophobic nitrogen-incorporated UNCD was synthesized in CH 4 /H 2 /O 2 /N 2 system without any post-treatment. • The ultra-smooth nitrogen-incorporated UNCD was obtained by equilibrating the etching effect of OH radical and growth promotion of CN and CH groups. • Hydrophobicity can be enhanced by raised hydrogen reconstructed sp 2 phase although the surface roughness was decreased. [ABSTRACT FROM AUTHOR]