Abstract: The errors for getting difference formulation from Maxwell equations in PML in 1 are indicated, and the correct derivation process through solving differen tial equation approach is presented. A second method directly yields different d ifference formulation based on the propagation characteristic of waves in PML me dium. The two different difference formulations are proved to be consistent. How ever, the latter is more accurate and easy to implement. Finally, a simple numer ical example validates that the absorbing performance is identical from the two difference formulations.