Excessive vibration in large-capacity turbine generators can lead to expensive repairs and downtime with associated loss of operational income. Therefore, it is vital to detect the rubbing location in the rotor system at an early stage. The challenge is in achieving this without the expense and labor of complete disassembly. In this study, we present a practical tool for estimating the rubbing locations in a large-capacity rotor system from the shaft vibration patterns at each bearing. The measurement and analysis were performed on the rotor system of an 800-MW class large-capacity turbine generator. Three vibration patterns were observed in the machine operating on-site and were explained in terms of vibration modal analysis and the unbalance response characteristics to identify the rubbing locations. These results will be of interest to industrial engineers and have the potential to increase savings in operation costs.