Boron nitride (BN) thin films were deposited on a Si(lll) substrate using ArF pulsed excimer laser radiation for ablating a hexagonal BN (hBN) target and a dc-gasdischarge for activating N-2. The present phases in the films were determined by Fourier transformation infrared spectroscopy (FTIR). The results show that BN thin films containing largely cubic BN (cBN) phase on Si(lll) substrates were successfully prepared by this dc-gasdischarge assisted pulsed laser deposition (PLD). The effects of the dc-gasdischarge and the chamber background gas on the formation of the cBN thin films were analyzed. The reasons for the phase evolution from hBN to cBN induced by the dc-gasdischarge are discussed. The results suggested that the dc-gasdischarge has two effects on the formation of cBN thin films. On one hand, it activated N-2 and caused the formation of nearly stoichiometric BN films. On the other hand, it caused the activated ions to bombard the substrate and induced the formation of the cBN phase. The dual beam ion source assisted PLD method is proposed to be the most promising method in preparing cBN films.