The induction melting was employed to prepare Nb-doped $CoSb_3$ skutterudites and their thermoelectric properties were investigated. Single phase $delta-CoSb_3$ was successfully obtained by induction melting and subsequent annealing at $400^{circ}C$ for 2 hrs in vacuum. The positive signs of Seebeck coefficients for all the specimens revealed that Nb atoms acted as p-type dopants by substituting Co atoms. Electrical conductivity decreased and then increased with increasing temperature, indicating mixed conduction behavior. Electrical conductivity increased by Nb doping, and it was saturated at high temperature. Maximum value of the thermoelectric power factor was shifted to higher temperature with increasing the amount of Nb doping, mainly originated from the high Seebeck coefficient around mixed conduction temperature and high electrical conductivity.