This paper investigates the root cause of a failure in gas-turbine blades, made of Nimonic-105 nickel-based superalloy. The failure was reported in two blades in the second stage of a turbine-compressor of a gas turbine in the hot section. Two failed blades were broken from the root and from the airfoil. The failure took place after 20 k h of service exposure in the temperature range 700–850 °C, with the rotating speed being in the range 15,000–16,000 rpm. The microstructures of the failed blades were studied using optical/electron microscopes. Energy dispersive X-ray spectroscopy (EDS) was employed for phase identification. Results showed that failure first initiated from the root. The dominant failure mechanism in the root was concluded to be corrosion-fatigue. The failure scenario was suggested based on the results obtained.