In the present study, a thermowell made of Incoloy 800 (33Ni-21Cr-46Fe) was used as a part to cover and therefore to protect a thermocouple inserted in a tube used in petrochemical process. It was then subjected to the atmosphere in the tube containing moving carbon particles at 900°C. One of the critical failure types observed was the combination of both solid particle erosion and oxidation. Thermowell degradation, which accounted for the corrosion-affected-erosion of materials, was significantly observed in the area where impingement at the head surface area of thermowell. The physicochemical characterization revealed that the thermowell consisted of two layers. The inner one is the incoloy 800 base metal, whereas the outer layer was clad with a cobalt base superalloy. Kinetic of oxidation of both materials in each layer, which was performed in air at 1173K presented the different parabolic rate. Metal oxide and surface morphology of thermowell, which was investigated in the simulated condition, were then examined by XRD, OM and SEM. A finite element software was furthermore applied to simulate the flow direction and the velocity of the particles. It was found that the maximum speed of particles (180 m s-1) was at the surface where the impingement angle of particle is 90°. Comparing to the failure of themowell in service, it was concluded that the failure of thermowell depends primarily on the impingement angle of particle. Design of the shape of thermowell, as well as the cladding with the higher erosion-corrosion resistance materials, were recommended to be carried out to improve the erosion-corrosion resistance of this part.