The effect of phosphorylated cashew nut shell liquid prepolymer (PCNSL), a phosphorus-based flame retardant (FR) possessing both hydrophobic and hydrophilic groups in the same molecule, on the processability, mechanical properties, thermal and flammability behavior, and aging characteristics of low-density polyethylene (LDPE) was studied. TGA showed that PCNSL enhanced the thermal stability of LDPE, whereas limiting oxygen index (LOI) values increased from 17 to 24.5. The large increase in the LOI value could most probably be due to the possibility of PCNSL acting, apart from being a normal phosphorus-containing FR, also as an antioxidant that might control the oxidative surface degradation of LDPE. The oxidative induction test (PE, 2.5 min, with 2% PCNSL, 225 min, and 2% Irganox(R), 189 min) showed that the antioxidant property of PCNSL is better than that of Irganox(R), a known antioxidant fur polyethylene (PE) and the enthalpy of oxidation (PE 4.6 kJ/g, with 2% PCNSL 2.6 kJ/g and 2% Irganox(R), 2.2 kJ/g) for the PCNSL/LDPE blend was less than that of LDPE and was comparable to that of the Irganox(R) system. Brabender Plasticorder traces of LDPE/PCNSL blends showed good processability and miscibility. The enhanced processability and miscibility between LDPE and PCNSL are attributed to the presence of the hydrophobic aliphatic segment in the FR, The SEM microstructure of the blend supported the above findings. Comparative evaluation with standard all-aromatic FRs such as decabromodiphenyl oxide (DBDPO) and tricresyl phosphate (TCP) was carried out and the data supported the finding that FRs having both aliphatic and aromatic moieties with hydrophobic and hydrophilic characters in the same system can give better property profiles than those without these attributes.