GaN based Schottky barrier diode (SBD) possesses advantages including high power density, high conversion efficiency, and excellent switching characteristics. During heteroepitaxial growth of GaN, a high density of threading dislocations is inevitably introduced, which can degrade device reliability. This paper reports a low dislocation density N⁺/N^– GaN quasi-vertical SBD fabricated on a freestanding GaN substrate. The characterization results of high-resolution X-ray diffraction and atomic force microscopy demonstrate that the high-quality epitaxial layer with a total dislocation density of 1.01 × 10⁸ cm^–2 and a root mean square surface roughness of 0.149 nm is achieved on a freestanding GaN substrate. The device prepared based on a high-quality epitaxial layer exhibits an ultra-low leakage current density of 10^–5 A/cm² at a reverse voltage of –5 V, without employing any edge termination structures or field plates or plasma treatment. Compared with the devices prepared on sapphire substrates using identical processes, the device prepared in this work reduces the reverse leakage current by four orders of magnitude. The experimental results show that the quasi-vertical GaN based SBD fabricated on a freestanding GaN substrate significantly reduces reverse leakage current and substantially enhances the overall electrical performance of the device. By employing emission-microscope (EMMI), leakage current in quasi-vertical SBD is identified to be primarily localized at the anode edge, and the underlying leakage mechanism is elucidated. Finally, temperature-dependent measurements demonstrate that the device maintains a leakage current below 10^–3 A/cm² at 100 ℃, confirming the potential of quasi-vertical SBD on freestanding GaN substrate for practical applications.