This Master Thesis analyze transient operating conditions of industrial steam turbine in few characteristic operating points. Within this work are given basics of heat transfer which were necessary for understanding heat transfer phenomena in steam turbine rotor. Today most used methods are given and described. Characteristic operating points covers all major changes of the steam flow as well as power output of steam turbine. Analysis of heat-stress load was done using FEM, finite elements method software. Precondition for this analysis was definition of thermal and mechanical boundary conditions upon which accuracy depends final results of analysis. Physical model of subject rotor is two-dimensionally organized axisymmetric one including technically approved simplifications and using specific finite element method software for analysis. Heat transfer boundary conditions are determined by means of correlations for heat transfer on particular parts of subject steam turbine rotor. For heat transfer calculation was required to develop thermodynamic and aerodynamic calculus for design output steam turbine operating condition as well as for transient operating conditions. Transient operating conditions are followed by significant space and time dependent fluctuations. Design output steam turbine calculation together with off design transient condition is given in this Master Thesis. Mechanical boundary conditions are determined by means of end user available documentation and from real machine operating data. Analysis of obtained results of temperature field and stress in the subject rotor as a result provided validation and verification of rotor design. Heat-stress load analysis results about steam turbine rotor during transient operating condition gives: satisfactorily support for further discussion about way of operating of the steam turbine considering parts of the rotor that are exposed to increased thermal and mechanical stress; information about susceptible rotor positions for crack initiation and development that may be qualified for further non destructive testing checks during regular maintenance activities in outages.