Abstract <p>In this study, boron was added to urea-formaldehyde resin’s structure, dry spinning and heat curing were then used to create boron-modified urea-formaldehyde fiber (BUFF). Which improved the fracture strength and toughness of urea-formaldehyde fibers. The chemical and morphological characteristics of the resultant fiber samples were investigated utilizing Fourier transform infrared spectroscopy (FTIR) and ¹³C nuclear magnetic resonance analysis (¹³C&#xa0;NMR). The thermal stability properties of various fibers were examined through thermogravimetric analysis (TGA), while their tensile strength was assessed using a tensile tester. The results demonstrated that boron was incorporated into the urea-formaldehyde molecules via B‒O bonds, which subsequently enhanced the three-dimensional network cross-linking within the fibers. This incorporation significantly improved the mechanical properties of the urea-formaldehyde fibers. The breaking strength of urea-formaldehyde fibers, thermally cured at 180°C for 20 min with the addition of 4% boric acid, was determined to be 481 MPa, reflecting a 138% enhancement over the original strength. The elongation at break was measured at 7.0%, indicating a 94% improvement from the initial value. Moreover, the thermal properties of the fibers were significantly improved, evidenced by a 20°C increase in the onset decomposition temperature and a residual carbon content of 28.47%.</p>