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Boron modified high-ortho phenolic fibers (o-BPFs) were prepared by melt-spinning from boron modified highortho phenolic resins (o-BPRs) with the weight-average molecular weight of 4973 g/mol, followed by being cured in a solution of formaldehyde and hydrochloric, and then heat-treated under high temperature. Gel permeation chromatography (GPC) and nuclear magnetic resonance spectroscopy (NMR) were used to measure the average molecular weight and ortho/para (o/p) ratio of o-BPRs. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to characterize the chemical and morphological structures of o-BPRs and o-BPFs. Thermogravimetric analysis (TGA) was employed to examine the thermal stability properties of different resins and fibers and the tensile strength of fibers was measured by a tensile tester. It was found that under proper curing and heat-treatment conditions, the tensile strength of o-BPFs reached 213.6 MPa and the char yield in N2 atmosphere at 800 °C attained 75.4 %. Compared with phenolic fibers (PFs), the decomposition temperatures at 5 % weight loss of o-BPFs in N2 and air atmospheres were increased by 156.8 °C and 219.0 °C, respectively.