Ni-doped $CoSb_3$ was prepared by the hot pressing, and its doping effects on the thermoelectric properties were investigated. Single phase ${delta}-CoSb_3$ was successfully obtained by encapsulated induction melting and the subsequent heat treatment at 773 K for 24 hrs, followed by the hot pressing under 60 MPa at 773 K for 2 hrs. Nickel atoms acted as electron donors by substituting cobalt atoms. Thermoelectric properties were remarkably improved by the Ni doping.