Orthorhombic boron nitride (o-BN) films with various thickness (150, 220 and 300nm etc.) are prepared on Si(100) substrate by radio frequency plasma enhanced pulsed (Nd:YAG) laser deposition (RF-PEPLD) in Ar-N2 gas system. The films are characterized by Fourier transform infrared spectroscopy and atomic force microscopic. The field emission characteristics of the BN thin films are measured in an ultrahigh vacuum system. A threshold electric field of 8V /6m and the highest emission current density of 157.5mA/cm2 at an electric field of 20V/6m are obtained for the 150nm-thick BN film and a threshold electric field of 18V / 6m and the highest emission current density of 332.9mA/cm2 at an electric field of 52 V/6m are obtained for the 220nm-thick BN film. The results show that the threshold electric field increases with increasing thickness of the films, while the withstand voltage characteristic of the BN films also increases with increasing thickness of the films. The Fowler-Nordheim plots show that electrons emitted from BN to vacuum by tunneling through the potential barrier at the surface of BN thin films.