We have theoretically studied the velocity-field characteristics in zinc-blende (zb) GaN by solving the Boltzmann transport equation with the help of one-particle Monte-Carlo method. The deformation potential acoustic phonon, polar optical phonon, alloy, impurity and intervalley phonon scatterings have been included in our computations. At an electric field of approximately 50 kV/cm, the negative differential resistance (NDR) has been observed at temperature T=300K. We have found that the self-oscillating frequencies of zb GaN diodes are 2.62 THz, 1.97 THz and 1.57 THz corresponding to diode lengths 0.3□ m, 0.4□ m and 0.5□ m respectively. ©2009 CODEC.