Abstract: Co_1-x-yNi_x+ySb_3-xSn_x polycrystals were fabricated by vacuum melting combined with hot-press sintering. The effect of alloying on the thermoelectric properties of unfilled skutterudite Co_1-x-yNi_x+ySb_3-xSn_x was investigated. A leap of electrical conductivity from the Co_0.93Ni_0.07Sb_2.93Sn_0.07 sample to the Co_0.88Ni_0.12Sb_2.88Sn_0.12 sample occurs during the measurement of electrical conductivity, indicating the adjustment of band structure by proper alloying. The results show that alloying enhances the power factor of the materials. On the basis of alloying, the thermoelectric properties of Co_0.88Ni_0.12Sb_2.88Sn_0.12 are improved by Ni-doping. The thermal conductivities of Ni-doping samples have no reduction, but their power factors have obvious enhancement. The power factor of Co_0.81Ni_0.19Sb_2.88Sn_0.12 reaches 3.0 mW·m^-1·K^-2 by Ni doping. The dimensionless thermoelectric figure of merit reaches 0.55 at 773 K for the unfilled Co_0.81Ni_0.19Sb_2.88Sn_0.12.