The influence of N2 additions (5%–40% N2/CH4) on various features of hot‐filament chemical vapor deposition (CVD) of diamond layers is presented and discussed using experimental results and calculations of the thermodynamic equilibrium. Small N2 additions (5%–10% N2/CH4) improved the diamond phase purity but led surprisingly either to an increase or decrease of the growth rate depending upon the filament temperature. The effects are attributed to a reduction of carbon supersaturation due to an abstraction of adsorbed hydrogen atoms caused by CN and HCN. Higher N2 additions (20%–40% N2/CH4) revealed a deterioration of the diamond phase purity and reversal growth rates. These results are probably caused by a beginning reconstruction of the diamond surface which is originated from enhanced abstraction of adsorbed hydrogen and the inefficiency of CN or nitrogen species to stabilize the diamond surface structure.