The behavior of transmission line under three-dimensional seismic excitations is studied by numerical simulation. According to a practical engineering, the transmission towers are modeled by frame elements and the transmission lines are modeled by cable element account for the nonlinearity of the cable. The effects of single-dimensional, two-dimensional and three-dimensional ground motions on the responses of transmission line are investigated using nonlinear time history analysis method, respectively. The results indicate that the longitudinal maximum response of transmission lines can be obtained considering longitudinal ground motion excitation only. The transverse maximum response of transmission lines can be obtained considering transverse ground motion excitation only. Neglecting multiple nature of ground motion in analysis will significantly underestimate the vertical responses of the transmission lines. To obtain an accurate seismic response of transmission lines, three-dimensional ground motion inputs are required.