Given the current abundance of waste plastics and the new technology to convert them into highly marketable products, waste plastics are projected as an alternative feedstock for producing some key organic products such as profitable alpha-olefins and paraffins through thermal cracking. This work was carried out to investigate the kinetics and to model the thermal cracking of waste plastics derived oil in a downer reactor by means of computer simulation. Attention was focused on the formation of the model for the waste plastic oil conversion, taking into account the process reaction kinetics, fluid mechanics, and heat transfer in the gas-solid downflow system, and the subsequent investigation of the model predictions under various conditions. Simulation results show that the downer reactor can achieve a high yield of the desired Medium Ends (ME) containing significant amount of alpha-olefins under a short residence time for a given conversion of liquefied waste plastics.