High Density Polyethylene (HDPE) was functionalized in an inert atmosphere at 160°C with two different amino esters at different experimental conditions by thermolysis method and the order of the functionalization reaction was determined. Analytical tools such as FTIR, DSC and TGA were used to characterize the HDPE functionalized with amino esters. The order of functionalization reaction was determined by FTIR-RI based kinetic method. The FTIR-RI kinetic results were critically compared with the literature values. A plausible reaction mechanism was used to explain the experimental results obtained.