A nitrogen-doped nanocarbon film electrode with mixed sp(2) and sp(3) bonds formed using the electron cyclotron resonance (ECR) sputtering method was studied with respect to the relationship between nitrogen concentration and electrochemical performance. The film (N-ECR) has a nanocrystalline structure, and the sp(3) content increases with increasing nitrogen concentration unlike the recently reported nitrogen-containing tetrahedral amorphous carbon film.1 The film has a very smooth surface with an average roughness of 0.1 to 0.2 nm, which is almost independent of nitrogen concentration. In contrast, the ratio of nitrogen-containing graphite-like bonding is high at low nitrogen concentrations, and then pyridine-like bonding increases as the nitrogen concentration increases. These variations in the chemical structures and the sp(2) and sp(3) content greatly change the electrochemical performance. The N-ECR electrode shows a wider potential window (∼3.8 V) than a pure nanocarbon electrode (∼3.1 V) due to its higher sp(3) content. The N-ECR electrode (N = 9.0 at. %) shows improved electrochemical activity because the lowest peak separation of Fe(CN)6(3-/4-) was observed at this nitrogen concentration. The oxygen and hydrogen peroxide (H2O2) reduction potentials at the N-ECR electrode shifted about 0.3 and 0.15 V, respectively, and the peak height of H2O2 is greatly increased. As a result, a linear relationship was obtained from 0.2 to 17 mM for the reductive current detection of H2O2. The N-ECR electrode also shows better activity for oxidizing certain biomolecules. The oxidation potentials of guanosine and adenosine decreased about 0.1 V, suggesting that the N-ECR electrode is suitable for use as a biosensing platform.