The structure and dynamics of magnetic vortices in high-T c superconductors are studied.The model we use is applicable to highly anisotropic high-T c superconductors, such as Bi and Th compounds, where the Josephson coupling is neglected between the superconducting layers. In this paper, we studied the ac losses occurring in a finite number of superconducting layers due to an applied ac current. It is found that the ac losses increase with the frequency and the amplitude of the applied current. By introducing random pinning centers in the layers, the effect of pinning on the reduction of ac losses is also studied. C8.13.2