Polycrystalline Cu1.8S composites dispersed with nanoscale (∼50 nm) and microscale (∼200 µm) SiO2 were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) method. The average grain size of the host Cu1.8S bulk is refined from 700 nm to about 500 and 50 nm by dispersing micro- and nano-SiO2 along with a fragmented SiO2 particle itself from the raw size of 200 µm and 50 nm to 2 µm and 20 nm, respectively. The Seebeck coefficient increases by dispersing SiO2 particles in the whole measuring temperature interval and reaches the peak value 88 µV K−1 at 623 K for the nano-SiO2 dispersed sample. The thermal conductivity κ ranges from 1.14 to 1.54 W m−1 K−1 by raising the temperature from 323 to 623 K for the nano-SiO2 dispersed sample, which is lower than that of pure Cu1.8S owing to the strong phonon scattering. The micro-SiO2 dispersed sample has a high κ, ranging from 1.6 to 2.2 W m−1 K−1 at the same temperature range due to the large intrinsic κ of micro-SiO2 particles. A maximum thermoelectric figure of merit value of 0.28 was attained at 623 K in the nano-SiO2 dispersed sample.