In previous research, we found that dimethyloldihydroxyethyleneurea (DMDHEU) is able to covalently bond hydroxy-functional organophosphorus oligomer (HFPO) to nylon and to cotton by formation of a crosslinked polymeric network. In this research, we applied HFPO to two nylon/cotton blend military fabrics using DMDHEU as the bonding agent. The treated fabrics showed high levels of flame retardant performance and excellent laundering durability. The treated fabrics passed the vertical flammability test after 40-50 laundering cycles. The DMDHEU concentration used in a finishing solution plays a critical role in determining the flame retardant performance and physical properties of the treated fabric. Increasing the HFPO/DMDHEU ratio increases phosphorus concentration after one laundering and reduces the fabric stiffness, but it also reduces hydrolysis resistance of the HFPO on the nylon/cotton blend fabrics. We also found that the fabric treated with HFPO and DMDHEU suffers little tensile strength loss. The pH of a finishing solution has no significant effect on the flame retardant performance and tensile strength retention of the treated nylon/cotton blend fabric in the pH range from 2 to 6.