Plastic pyrolysis is widely studied and implemented at lab-scale but rarely modelled numerically. For the sake of designing efficient industrial reactors, modelling plastic pyrolysis process at particle scale could be a prerequisite. Therefore, the aim of this work is to model the whole plastic pyrolysis process, on particle scale, for polypropylene (PP) and high density polyethylene (HDPE) inside a thermogravimetric analyzer coupled with differential scanning calorimeter (TGA-DSC). First, the kinetic triplet for PP and HDPE pyrolysis, at heating rates 4-10°C/min, are determined according to the isoconversional methods. Secondly, the kinetic triplets are used along with the appropriate conditions to model and simulate the pyrolysis process for PP and HDPE in TG analyzer, using finite element method. The melting sub-process is modelled using the modified apparent heat capacity method, which resulted in a relative error below 10% between the simulated and measured heat flow. Furthermore, the cracking model describes perfectly PP and HDPE cracking, where the average relative error among all calculated and experimental conversions didn't exceed 5%. Furthermore, the heat flow inside the TGA-DSC crucible was modelled and the average error was reduced to less than 8% by dividing the cracking phenomena into a “latent” and an apparent process.