Purpose: This study sets out to determine the characteristics of high-temperature creep resisting steels under conditions of thermo-mechanical fatigue with the use of a method proposed in the Code-of-Practice under the EU TMF-Standard project. Design/methodology/approach: The thermo-mechanical fatigue (TMF) tests were carried out in the conditions where the value of complete strain and the temperature were under control. Two methods of investigating samples in TMF tests were applied: OP (out-of-phase) and IP (in-phase). Findings: Based on the tests, the characteristics of TMF life was determined and it was found that X20CrMoV12.1 steel shows lower life in comparison with new steels: X10CrMoVNb9-1 (T/P91) and X10CrWMoVNb9-2 (T/P92). The results of the OP tests made for X10CrMoVNb9-1 (T/P91) steel are an exception here. Tests of thermo-mechanical fatigue have shown that in a majority of cases in fatigue tests, the X20CrM0V12.1 steel has lower TMF life when compared to X10CrMoVNb9-1 (T/P91) and X10CrWMoVNb9-2 (T/P92) steels, despite its better strength properties, as a measure of which, the range of stress was adopted. Research limitations/implications: At the present stage of the research, two types of tests (IP and PO) were performed. Due to a limited number of experiments connected with the application of selected types of tests and their number, the conclusions resulting from the research may, at the present stage, serve as guidelines for its continuation only. Practical implications: The test results may also be used to compare the properties of creep resisting steels used in the power engineering industry and represent a contribution to widening the knowledge of the behaviour of materials under thermo-mechanical fatigue conditions. Originality/value: This study is one of the first attempts to determine the TMF life characteristics of the steels used in the Polish power engineering industry.