In order to reduce the vibration of the stator core in the large hydro-generators, the recent tendency is to stack the stator core at site for the hydro-generators with the diameter larger than ten meterws. Owing to the temperature difference between the core and frame of the stator, the frame will restrain the expansion of the core, which will lead to buckling of the stator core in case of high-stiffened frame. But in order to prevent forom the eletrmagnetic vibration the stator frame will be required to have an appropriate supporting stiffness for the stator core. The investigation of the thermal deformation in the generator must be based on comprebensicve analysis of the temperature distrbution within the begnerator under different operation conditions. Inthis paper a method by FEM for calculation of the 3D thermal field within the benerator is applied. Based on the results of the thermal field calculation, the thermal deformation of the stator core and the frame can be calculated. As an example, the thermal field and deformation of a 400MW evaporating cooled generator have been calculated. The comparison of the calculated results with those measured at site proves that the applied method in this paper has apprpiate precision.