Developing full-wave simulators for high-frequency circuit simulation is a topic many researchers have investigated. Generally speaking, methods invoking analytic pre-processing of the device¡¦s V-I relations (admittance or impedance) are computationally more efficient than methods employing numerical procedure to iteratively process the device at each time step. For circuits providing complex functionality, two-port or possibly multi-port devices whether passive or active, are sure to appear in the circuits. Therefore, extensions to currently available full-wave methods for handling one-port devices to process multi-port devices would be useful for hybrid microwave circuit designs. In this dissertation, an efficient scheme for processing arbitrary multi-port devices in the finite-difference time-domain (FDTD) method is proposed. The device¡¦s admittance is analytically pre-processed and fitted into one grid cell. With an improved time-stepping expression, the computation efficiency is further increased. Multi-port devices in the circuit can be systematically incorporated and analyzed in a full-wave manner. The accuracy of the proposed method is verified by comparison with results from the equivalent current-source method and is numerically stable.