Photonic crystal fibers (PCFs) offer new possibilities of realizing highly birefringent fibers due to a higher intrinsic index difference and flexibilities of fabrication process compared to conventional fibers. A kind of highly birefringent index guiding PCF with a full vector model is analyzed theoretically. Two-fold rotational symmetry was introduced into the fiber structure by creating a regular array of air-holes of two different sizes in its cladding. The degeneracy of two orthogonal polarized-modes of the fundamental mode is removed. This creates a possibility to attain a modal birefringence of at least one order of magnitude larger than for conventional high birefringence fibers. A beat length of 0.4067 mm is obtained at the wavelength 1540 nm. The numerical results are in very good agreement with the experimental results.