Applicability of multimode interference effect and self-imaging principle for terahertz waves in two-dimensional silicon photonic crystal waveguides are investigated by modeling and computation. The results show that the multimode interference effect and the self-imaging principle are applicable for terahertz waves. As an example, a splitter and a filter for terahertz waves have been proposed, calculated and analyzed by finite-difference time-domain method based on the multimode interference effect and the self-imaging principle.