This paper concentrates on the estimation of reliability of an automotive electronic power device using thermal cycle, aging effect and accumulated damage. Thermal model has been developed for analysing the factors of power loss. A look-up table has been constructed to calculate the power loss and finite-element modelling was used to find out the aging effect. Thermal model was developed by using FLOTHERM. It is in the form of either Foster or Cauer network. Comparison of results from the thermal model has been done. Preprocessing of data by filtration and spectrum truncation was also explored. Reliability can also be estimated by calculating the accumulated damage with the help of Pagoda Roof method and Miner’s rule. Finally, a method was proposed of using Ga NFET instead of MOSFET to calculate power dissipation.