The thermal degradation kinetics of nano-polystyrene particles with diameters of 60, 90, 160, and 225 nm were investigated in nitrogen atmosphere using thermogravimetric analysis (TGA). Various kinetic models were employed to determine the thermal degradation mechanism and kinetics. Nano-polystyrene particles have relatively lower thermal stability when compared to micro-polystyrene. Both differential thermo–gravimetric (DTG) data and apparent activation energies indicate that the thermal degradation of nano-polystyrene particles at 60 nm is a two-step reaction process where the second step plays a dominant role, while nano-polystyrene particles with diameter greater than 60 nm exhibit single-step degradation. Similar to most micro/macro polystyrene particles, DTG peaks of nano-polystyrene particles shift towards higher temperatures with increasing heating rates. Thermal degradation of nano-polystyrene particles under nitrogen atmosphere follows the first-order reaction model. However, the apparent activation energies increase (162-181 kJ·mol^–1) with the increase of particle sizes (60-225 nm). This study could provide some insights into pyrolysis of nano-polystyrene particles and a safer process of manufacturing, storage and handling of nano-polystyrene particles.