Cobalt monosilicide (CoSi) prepared by arc melting possesses a very high thermoelectric power factor of 60 mu W cm(-1) K-2 at room temperature. The high lattice thermal conductivity of CoSi, however, limits its thermoelectric efficiency. In this paper, powder processing technique was intentionally used to reduce the grain sizes of CoSi and thus suppress its lattice thermal conductivity. The effects on the thermoelectric properties of CoSi under various processing conditions were explored. The electrical resistivity is drastically increased, while the absolute Seebeck coefficient is decreased for the powder processed samples compared to the arc-melted one. The detrimental influence is attributed to the grain boundary defects produced during processing, which brings about an excessive energy filtering effect and hence induces a huge decrease in the absolute Seebeck coefficient. The lattice thermal conductivity of powder processed samples is reduced as expected. However, the reduction is always offset by the reduction in the mobility due to the similar mean free paths of electrons and phonons of CoSi. As a result, it may not be feasible to upgrade the thermoelectric performance of the CoSi system by powder processing.