The work aims to investigate the reasons for surface cracking of a first stage rotor blade in a heavy duty gas turbine, which has been in service for about 80 000 h. The microstructures and corresponding element distribution at the critical positions of the cracked blade were determined with field emission scanning electron microscope (FESEM) and energy disperse spectroscopy (EDS). The microstructure evolution both at the surface position of the coating and at the interface between the coating and the substrate of blade caused by the high-temperature oxidation in the service process were revealed. A coating with single-layer structure and corrugated surface morphology was observed on the surface of the blade. The results of EDS analysis confirmed that this coating with high contents of elements Co, Ni, Cr and Al was the anti-oxidation coating (or named as bonding coating) rather than thermal barrier coating. Besides, the protective thermally grown oxide layer which was usually formed during high-temperature service was not observed on the top surface of anti-oxidation coating for the blade. It could be inferred that the thermal barrier coating as well as the thermally grown oxide layer on the top surface of the blade fell off in the service process. The anti-oxidation coating was NiCoCrAlY alloy with a mixture microstructure of γ-Ni+β-NiAl phases, while the blade substrate was GTD-111 nickel-base superalloy with a mixture microstructure of γ-Ni+γ?-Ni3(Al, Ti) matrix, γ/γ? eutectic and (Ti, Ta)C carbides. The surface cracks were mainly distributed at the transition platform between the airfoil and the root section of the blade. In addition, the cross sectional morphology indicated that the surface cracks originated from the thermal erosion pits which formed on the surface of the coating in the long-term service process, and then gradually grew into the interior in the following high temperature service process. Severe metal oxidation in the inner of cracks demonstrated that the cracks facilitated the inward diffusion of oxygen and accelerated the oxidation of metal elements. The accumulation of elements sulfur and vanadium which mainly came from the corrosive contaminants contained in the air and fuel was detected in the inner of the cracks. Furthermore, plenty of oxides were also found in the inner of the anti-oxidation coating and at the interface between the coating and blade matrix. The oxidation products were mainly composed of oxides containing elements Al and Cr. The oxidation of element aluminum during service induced the phase transformation from β-NiAl to γ-Ni at the surface position of the coating and γ?-Ni3(Al, Ti) to γ-Ni at the interface between the coating and the substrate of blade. The β-NiAl phase acted as a strengthening phase played a significant role in determining the performance of the coating. The formation of γ-Ni layer accompanying with the disappearance of β-NiAl phase on the top surface of the coating lead to the deterioration of mechanical properties. It can be concluded that the disappearance of the high hardness β-NiAl phase combining with the surface hot-corrosion pits and stress concentration result in the formation and propagation of cracks at the transition platform between the airfoil and the root section of the blade.