The finite-difference time-domain (FDTD) method is a popular time-domain numerical method for solving RF/microwave structure problems. It can compute the wideband electrical parameters such as S-parameters. In simulating a waveguide structure, effective absorbing boundary conditions are needed to terminate the waveguide while simulate the infinite long waveguide that create no reflections of modes. To address the issue, many absorbing schemes were proposed including the perfectly matched layer (PML) and one-dimensional modal absorbing techniques. However, they are derived analytically or semi-analytically, and therefore they present the errors or reflections that are not negligible, in particular, at or below cutoff frequencies. In this paper, we present our recent progress in developing high-performance absorbing boundary conditions for modeling waveguide structures: a one-dimensional modal FDTD method and a one-dimensional modal PML. They are derived directly from the FDTD formulation, and consequently they have numerical properties very close to the conventional FDTD method. When used as the absorbing boundary condition, they can achieve reflections of less than −200dB even at and below the cutoff frequencies. When used to obtain an incident wave in a waveguide, it can have the field solutions with difference of less than −200dB from those produced by the conventional FDTD method.