The transmission characteristics of perforated flat plates which act as dichroic filters or frequency selective surfaces were measured in the far-infrared (FIR) region by Terahertz Time-Domain Spectroscopy (THz-TDS). The mechanically made dichroic filters were designed for the cutoff frequencies of 155, 586, and 773 GHz. Due to their sharp cutoff characteristics, dichroic filters are used in quasi-optical systems as compact, high-pass filters throughout the sub-millimeter wave and FIR region. Knowledge of the frequency response of such filters is important for various applications in frequency mixing and frequency multiplying experiments in the microwave region. The spectrum of a freely propagating THz pulse with and without filter was measured by electro-optic sampling in a ZnTe crystal. The experimental results for the frequency dependence of the transmittance of the dichroic filters are in good agreement with calculations based on the theory given by Chen .