The effect of boron phosphate ($BPO_4$) nanoparticles on the mechanical, thermal, and flame retardant properties of polypropylene (PP) and polyamide 6 (PA-6) fibers are investigated by tensile testing, thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), and micro combustion calorimeter (MCC). The addition of $BPO_4$ reduces the mechanical properties of the both PP and PA-6 fibers. According to the TGA results, the addition of $BPO_4$ does not change the thermal behavior of PP fiber and slightly reduces the thermal stability of PA-6 fiber by about $30^{circ}C$. According to MCC results, the addition of $BPO_4$ does not change the effective total heat evolution and heat release rate (HRR) peak for PP fibers. Although the inclusion of $BPO_4$ does not change the total heat evolution of PA-6 fiber, it reduces the HRR peaks due to increase in barrier effect of char.