Gas turbine blade health monitoring is of critical importance due to the severe operating conditions and catastrophic failure consequences. This paper aims to develop a non-destructive vibration-based diagnostic index to investigate the effect of root crack growth on its natural frequencies. Using the finite element method in finite element software, a simplified 3D model of a gas turbine blade with Siemens V94.2 turbine operating conditions under realistic combined thermal-mechanical loading is created. Stress analysis in the uncracked state identified the blade-body connection area as the most susceptible area for crack initiation. In the next step, a programmatic modal analysis was performed for different depths of an edge crack from 0.5 to 4.5 mm. The results constructed a critical crisis at a depth of approximately 2 mm, such that the natural frequencies are mild before this point, and nonlinear and severe after it. Also, the vibration stories were different; Thus, the second mode with 25% reduction, the highest percentage of frequency reduction, was found to be the sensitive index for tracking the most advanced damage. These findings indicate that monitoring the change in natural frequencies, can be used as an efficient tool in predictions for crack detection and failure.