In this paper, a modified FDTD method using a hybrid Cartesian-cylindrical coordinate system is presented for modeling long wires with a axial symmetry structure. The wires are described with FDTD equations in a cylindrical coordinate system, and the non-wire zone in a Cartesian coordinated system. A thin-wire model is employed to link these field equations, and a time-domain solution is available. This modified FDTD method is applied to solve transient electromagnetic fields associated with long wires covered with insulation material. Reasonable result is obtained.