Abstract: The main difference between the discontinuous Galerkin time domain (DGTD)and the finite element time domain (FETD) is the exchanging field by numerical flux. Based on the weak form solution of TM case in 2D, the physical meaning of numerical flux and the different express between main unit and adjacent unit are firstly described. And then, combining the above, the DGTD iterative formulae are obtained. Finally, numerical examples of line source radiation and interference are given to demonstrate the validity of DGTD algorithm.