The current study deals with the effect of boron compounds, zinc borate (ZB), borax: boric acid combines (BxBc), and ulexite which are used to improve fire resistance, on the thermal, microstructural, and mechanical properties of the flax/PLA biocomposites. For this purpose, the mechanical properties of the biocomposites were determined by tensile, impact, and three-point bending tests. Thermogravimetric analysis and the UL-94 burn test were conducted to determine the thermal properties of composites, such as thermal stability and flammability. Thermo-mechanical properties and viscoelastic characteristics of biocomposites were determined by the dynamic mechanical thermal analysis method. Test results showed that ulexite addition could significantly reduce the burning rate by 80%. However, although mechanical properties were adversely affected by the incorporation of boron compounds resulting in a drop of up to 20%, it is believed that boron compound-filled flax/PLA composites have mechanical and thermal properties that can be considered as an alternative material, especially for the automotive industry.