Nanopowders with nominal compositions of Co4Sb11.5Te0.5 and In0.5Co4Sb11.5Te0.5 were prepared via hydrothermal synthesis at 180 °C for 48 h, then heat treated and finally hot pressed at 625 °C and 80 MPa for 1 h in vacuum to form bulk samples. The phase compositions of the samples were determined by X-ray diffraction. Hall Effect measurement of the samples was carried out at room temperature. The fracture surface of the samples was observed by field emission scanning electron microscopy. The electrical conductivity and the Seebeck coefficient of the samples were measured from room temperature to around 748 K. The In-filled and Te-doped CoSb3 sample with longer time annealing before hot pressing had much better electrical transport properties with the highest power factor of 38.4 μWcm−1 K−2 around 573 K.