FePt bit patterned media (BPM) was fabricated with a self-assembled polymer mask with a feature size of 12 nm pitch (equivalent to 5 Tdots/in2 ). A 3.5 nm FePt film with high c-axis crystal orientation was prepared for the magnetic recording layer. A solvent vapor annealing process was applied for obtaining uniform directed self-assembling of polystyrene (PS)-polydimethylsiloxane (PDMS) diblock copolymer pattern. Pattern transfer from a polymer mask to FePt layer was achieved by employing a carbon hard mask. In spite of excellent magnetic characteristics of FePt layer, the fabricated FePt BPM showed small coercivity (Hc) of 6 kOe and large switching field distribution (SFD) of 21%. These results are due to the etching damage of FePt dots. Disordering of FePt L10 phase by the etching damage reduced magnetic anisotropy energy (Ku). The damaged portion became a nucleus of the magnetization reversal and reduced Hc. Distribution of the damaged volume and the extent of the Ku reduction contributed to large SFD. This model is supported by the experimental data of magnetic field angle dependence of switching field. The result suggests the domain wall motion type of magnetization reversal mode, where the domain wall is created at the interface between the damaged portion and the internal high-Ku region.