First quantitative estimates are presented for nitrogen isotopic fractionation during diamond crystallization with respect to nitrogen-bearing fluid components using quantum-mechanical (DFT) calculations on the defect (with the substitutional nitrogen) diamond lattice. Provided equilibrium isotopic fractionation, 15N/14N ratio decreases within the sequence of compounds NH4+ > N2 > (diamond, NH3) > CH3N > CN− > NH2. At temperatures of 1,100 to 1,200 °C fractionation among diamond and fluid N-compounds are estimated at –2.23, –0.77, 0.01, 0.44, 1.31 and 2.85 ‰ and substantially (over 1 ‰) exceed the already available estimates based on the modeling diamond C-N bonds by analogy with HCN or CN^– molecules. Depending on the dominant nitrogen and carbon substance in the mineral-forming fluid, diamond formation can be accompanied by different isotope compositional trends, as expressed either by zoned patterns within individual diamond grains or by isotopic δ15N vs δ13C covariations during successive crystallization. Provided the dominance of NH3 component (the reduced conditions, high pressures and the cold geotherm) nitrogen isotope fractionation between diamond and fluid does not exceed 0.1-0.2 ‰ and the isotope shifts at temperature ca. 1100 °C Δ15N << Δ13C. In nitrogen depleted reduced mantle fluids possible existence of compounds with low heavy isotope affinity at temperature of diamond formation (especially NH2) implies high isotope fractionation between diamond and the fluid and hence, evolved Δ15N/Δ13C ratios. Oxidized fluids dominated by CO2 or CO3 coupled with N2 component are characterized by close to zero Δ15N/Δ13C ratios as inferred by prevailing carbon isotope fractionation with respect to nitrogen isotopes, the latter change considerably with nitrogen distribution coefficient among diamond and the growth media.