Time-of-flight mass spectrometric and optical emission spectroscopic studies have been carried out on ablation plumes produced by laser ablation of graphite at 266nm in vacuum with or without a magnetic filed of about 0.1 T. Some processes including ionization, cluster formation, and deexcitation were found to be enhanced by the magnetic field. We have also examined the influence of the magnetic filed on the deposition of carbon nitride films using laser ablation of graphite in a nitrogen atmosphere. Corresponding to an intense CN emission, a magnetic field induced enhancement of N incorporation and the formation of sp³ C-N bonding were both observed in the films. These results suggest a possible reaction control in ablation plumes of carbon and related materials by a magnetic filed.