The plasma-chemical deposition of diamond-like carbon (DLC) films onto heavily boron-doped single-crystal p-type diamond (the concentration ~1020 cm–3) in CH4 + Ar plasma is conducted. The deposition rate is 7 nm min–1. The elemental composition and properties of the films are studied in detail. It is found that the films are enriched with hydrogen, possess a density of 2.4 g cm–3, and exhibit an ultrasmooth surface (with a roughness of 0.4 ± 0.2 nm).