Polycrystalline silicon sample was prepared by crushing, ball-milling and hot-pressing a single crystalline silicon wafer. The polycrystalline product was characterized with optical microscopy, TEM and SEM. Its porosity was found to be 15-22%.. The following thermoelectric properties of both the single crystal silicon and the polycrystalline sample were determined: thermal conductivity, electrical resistivity, Hall mobility, carrier concentration and Seebeck coefficient. It was found that porosity and polycrystallinity decrease thermal conductivity only slightly. They also decrease the Seebeck coefficient, carrier concentration, carrier mobility and electrical conductivity.Literature on effective medium theory (EMT) is reviewed. It is discovered that the apparent validity of an effective medium theory depends strongly on the model that is chosen for description of the composite. Careful microstructural characterization is necessary for successful use of an EMT. A new way of studying porosity in the context of EMT is suggested.