The influence of oxidation time on the microstructure and mechanical property, especially on the hardness, and of Co-base alloy coatings were investigated. Coatings were manufactured by PTA cladding. The cobalt alloy was exposed to temperature 800°C in air for 1, 22 and 200 hours. As a result of oxidation treatment the scale layers were formed and changes in microstructure was observed. Also changes in hardness were noticed.