The purpose of this study is to investigate the role of filling materials (e.g., quartz sand and kaolin clay) in the interaction between a P-wave and a rock fracture. The specific fracture stiffness reflects the seismic response of a filled fracture, while the wave transmission coefficient describes P-wave transmission across the filled fracture. A series of experimental tests using a split Hopkinson rock bar technique are conducted on artificial rock fractures that are filled with pure quartz sand, sand-clay mixtures with 30%, 50% and 70% clay weight fractions, and a pure clay matrix. The boundary conditions of the filled fracture, i.e., the displacement and stress discontinuities, are used in the method of characteristic lines to calculate the wave transmission coefficient in the time domain. The analytical results agree well with the experimental results. The specific fracture stiffness and the wave transmission coefficient decrease with increasing filling material thickness. When the clay matrix completely fills the void space of the quartz sand, the filled fracture exhibits the largest specific fracture stiffness and promotes P-wave transmission. In general, the wave transmission coefficient is strongly related to the specific fracture stiffness, regardless of the filling material composition or the filling material thickness. (C) 2014 Elsevier B.V. All rights reserved.