As a fundamental measurement unit in power grids, the current transformer plays a significant role in electrical energy metering. As a result of the wide application of nonlinear loads and power recourses in power grids, such as power electronics, green energy generation, etc., undesired DC- current components, in many occasions, present in power lines. A DC bias current can significantly lower the measurement accuracy of a current transformer and, in heavy cases, can saturate the magnetic core of the current transformer. Therefore, it is necessary to evaluate the performance of the current transformer under DC bias currents. Different from the conventional equivalent circuit model of an instrument transformer, we present in this research a magnetic analysis of current transformers under the DC bias condition. An analytical model that links the measurement error to experimental parameters, e.g. ampere-turns, dimensions and material of the magnetic core, load in the secondary winding, etc. The finite element analysis is employed to verify the analytical model. The performance of a typical current transformer with DC and AC is discussed.