Finite element analysis could achieve a near perfect result in computing the temperature field and stress field of steam turbine rotor in operation. This paper adopts the input and output data of finite element analysis under cold startup process to establish an online damage monitoring model of a steam turbine rotor. The sensitivity of thermal stress transfer function is analyzed using Nyquist plot. Comparison between finite element analysis model and this proposed model validates the accuracy of this work in assessing the low cycle fatigue damage. The present paper introduces a feasible online damage-monitoring methodology of steam turbine rotor in engineering. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.