We present first-principles energetics of a phosphorus (P) impurity atom substitutionally doped in the vicinity of the C(001)-2×1:H and C(111)-1×1:H surfaces. We find that locating the P atom within topmost four C-atomic layers of the C(001)-2×1:H surface substantially lowers the formation energy relative to that for doping in the bulk, where the energy lowering (Δ E_form) is up to ∼4 eV. For C(111)-1×1:H, the Δ E_form is up to ∼2.5 eV. Thus we expect that the doped P atoms may have a strong tendency of segregation close to these surfaces. We will also discuss an implication of our result in the context of a recent experiment of P-doping of the (001) and (111) oriented diamond films [H. Kato, S. Yamasaki and H. Okushi, Appl. Phys. Lett. 86, 222111 (2005)]. [DOI: 10.1380/ejssnt.2006.124]