Due to high wear resistance and chemical stability at high temperature, cobalt base alloys have presently been used as materials for hardfacing in several applications, for example, coating a head of thermowell, a protecting part for thermocouple, in petrochemical production. One of various techniques used for hardfacing is often weld cladding by using a gas tungsten arc welding (GTAW) method. However, welding parameters should be exactly controlled in order to obtain desired materials due to their structure. The objective of this experiment is to study the influence of the GTAW parameters on the high-temperature erosion-corrosion resistance of Incoloy 800 thermowell cladded by the cobalt-base alloy filler, Stellite12 (Co-29.5Cr-8.5W-1.4C) at 900°C. The studied parameters were welding speed in the range of 0.1-0.9 mm s-1, welding current in the range of 60-75 A, and flow rate of pure Ar used as shield gas in the range 3-20 liter min-1. The results showed that when erosion rates had a tendency to increase with increasing current for all welding speed except 0.8 mm s-1. The erosion rate decreased with the increasing current for the welding speed of 0.8 mm s-1. Using flow rate of 5 liter min-1 induced the lowest erosion rate. The optimum GTAW parameters giving the lowest erosion rate were the welding current of 75 A, flow rate of 5 litter min-1, and welding speed of 0.8 mm s-1. However, it was found that hardness results did not correspond to the erosion rate. It was found that high hardness led to heavy mass loss due to the fracture on the surface. From metallographic observation, the oxidation could be observed on the sample surface before the erosion testing. However, the spallation of oxide scale might be involved due to the high erosion rate. It was also observed that the solid solution phases influenced more significantly on the erosion - corrosion resistance than the carbide did.