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This paper regards control and optimization of a gas turbine operation with a respect to effectiveness and reliability criteria. The development of diagnostics procedures begins with modeling of the gas expansion line for various loads with regard to an internal blade cooling. Analyzing an expansion line allows to assess the influence of cooling parameters on gas path and turbine operation. Computational models are described here, which respect various cooling aspects, mainly convection and film cooling. Several examples of gas path, blade temperature and thermal barrier coating (TBC) temperature dependencies on cooling parameters are presented. The determination of gas and metal temperatures is utilized to evaluate creep wear rate. The dependency between the strength and durability of turbine components and their temperature might be regarded in several aspects. Firstly: lower temperature decreases the creep rate. Even small temperature drop leads to a significant deceleration of creep processes. Secondly: the change of the temperatures modifies also the distribution of thermal stresses. This change of thermal stresses is usually very small and might be neglected. Presented analysis omits the influence of temperatures alteration of stresses distribution, it regards however the change of the creep characteristics.