Abstract: Based on the finite-difference time-domain (FDTD) method in anisotropic medium and nearly perfect matched layer (NPML) theory, the implementation of the modified NPML absorbing boundary condition (ABC) for truncating anisotropic medium is presented. By inserting the stretching factor within the space partial derivatives of Maxwell's curl equations and combining the spatial interpolation method, the ABC formulations method is derived. The associated expressions are easy to realize in the FDTD method. The reflection errors of the radiation fields of an electric dipole are calculated and thus the validity of the proposed ABC for truncating 2-D anisotropic medium is verified. In 3-D examples, the radiation phase distribution of the time-domain field as well as the time-dependent relative error for different layers NPML absorbing boundary are simulated with the algorithm. Numerical results demonstrate that the NPML ABC has good absorbing performance for electromagnetic wave in anisotropic mediums.