The Cu-Fe-S system consists of earth-abundant low-toxicity elements, and it potentially has good thermoelectric properties. There are many different types of Cu-Fe-S compounds, and some are n-type semiconductors while others are p-type semiconductors. Here, we report a rapid low-cost colloid-chemical method to fabricate Cu-Fe-S thermoelectric materials using Cu2S and FeS nanoparticles as building blocks. The n- and p-type of the Cu-Fe-S nanobulk material can be readily tuned by changing the Cu2S/FeS volume ratio. The nanobulk materials lead to low lattice thermal conductivity ranging from 0.3 to 1.0 W m(-1) K-1. By blending Cu2S and FeS nanoparticles at a weight fraction of 9:1, we fabricated a nanocomposite consisting of bornite Cu5FeS4 as the main phase and other minor phases. This nanocomposite has a maximum dimensionless figure of merit (ZT) of 0.55 at 663 K, which is 45% higher than that of pristine Cu5FeS4.