The electrical properties of NiO-YSZ composites were investigated by the measurement of a. c impedance electromotive force of galvanic cell and oxygen partial pressure dependence of electrical conductivity between 23$0^{circ}C$ and 100$0^{circ}C$. Samples of 19 compositions were sintered at 1$600^{circ}C$ for 4 h from NiO-YSZ powders made by solid state reaction. No other phase except NiO and YSZ was discovered by XRD and the solubility of NiO in YSZ was less than 10 mol% and that of YSZ in NiO was less than 1 mol% The large variation of the electrical conductivity with composition at 30$0^{circ}C$ decreased with increasing temperature. When x in xNiO-(1-x)YSZ was equal to or less than 0.4, electrical conduction was mostly dependent on ionic charge carriers of YSZ and when x was equal to or greater than 0.8 the conduction was mostly dependent on electron holes of NiO Mixed conduction was observed for 0.4<x<0.8