The 3D stator temperature fields of a SF300-48/1230 hydro-generator are calculated, using the isoparametric element method of hexahedron and eight nodes. The wires insulation aging and shelling in the generator is taken into account, and the fluid similarity theory is applied to calculate temperature fields. The computed results show good agreement with the measured results. Based on the results, some useful conclusions are drawn that provide a theoretical basis for real-time temperature monitoring and fault diagnosing for generators.