Abstract—A novel Porous Core Photonic Crystal Fibre (PC-PCF) with high birefringence and extremely low transmission loss is proposed and investigated. High birefringence is achieved by using high resistivity float zone silicon as background dielectric material. Simulation results demonstrate that the proposed PC-PCF has a birefringence above 0.9, Effective Material Loss (EML) of 0.012 cm-1 and 0.0038 cm-1 and confinement losses in the order of 10-19 and 10-7 for X- and Y-orthogonal fundamental modes, respectively, at frequency 1.3 THz. It is shown that whilst maintaining its high birefringence, the X-polarisation mode shows low total losses in a wider frequency range than the Y-polarisation mode. Moreover, variation of the core power fraction and effective modal area with frequency is carefully studied. The proposed PC-PCF comprises of simple geometry in both core and cladding and thus can be easily fabricated using existing fabrication technologies. Its high birefringence makes it potentially suitable for integration in applications such as terahertz medical imaging, terahertz filtering, terahertz interferometry, and sensing.