Abstract: The transfer characteristics of current transformers （CT） will subject to possible changes due to DC bias, which may thereby bring about severe impacts on reliability of the relay protection and fairness of the power metering. Based on equivalent circuit models and electromagnetic equations, the integrated effects of both DC bias and remnant flux on the time-to-saturation of protective CTs were deduced, and thereby numerical expressions were quantitatively given. Mathematical formulas of phase-displace- ment and ratio error were also obtained for metering CTs with DC bias. Analysis indicates that, the DC bias current will accelerate saturation of the protective CTs up to several milliseconds, and also increase transfer errors of the metering CTs. Simulation and case study were given to verify the above analysis results. The proposed research presents referential basis for proper selection of both protective and metering CTs as well as reliable setting of corresponding relay device in power systems.