We investigate the annihilation dynamics of the disclination loop in bulk nematic liquid crystals. This work is based on the Frank free energy and the nematodynamics equations. The dissipation of energy is calculated by two methods. In the first method, the dissipation of energy is obtained from the Frank free energy. In the second method, it is calculated by the nematodynamics equations. Finally, we obtain the equation of motion and annihilation velocity of the disclination loop.