The electric dipole excitations are investigated in the framework of the self-consistent HF + RPA approximation with Skyrme interactions. Magic O, Ca, Ni and Sn isotopes are used to investigate low-lying modes. Neutron and proton transition densities analysis of each nucleus is provided to examine the behavior of Pygmy Dipole Resonance (PDR) of nuclei under excitation, showing a complex pattern along the nuclear chart. The PDR in the very neutron-rich Ca-70 nucleus exhibits a novel mode of excitation, the so-called arch mode. The collectivity of the PDR is also analysed. (C) 2012 Elsevier B.V. All rights reserved.