The research activity was carried out on two types of modern steels: - high yield stress structural steels and - steels with increased creep deformation resistance for thermal power works equipment. The cooperation of members of the program group with industrial companies producing steels and companies, which have used and are projecting to use structural steels for different purposes, has achieved a considerable level. They are involved also with on site work for the evaluation of existing structures and equipment. For reliable evaluations knowledge and experience gathered from own research are needed beside informations disponible in scientific and technical literature. Only a proper combination of both kinds of informations can be a reliable base for clear answers to questions, as f.i. what kind of steel would give the best combination price-applicability for a determined structure or equipment part or what is the safe residual life of a steam boiler part. The experimental work is specific and it requires sometimes a long preparation of specimens, f. i. an annealing up to two years is necessary for the understanding the evolution and the effect of brittleness of NiCrMo cast stainless steel used for parts in nuclear power works or for the determination of the correlation microstructure-creep rate for steels for thermal power works parts.Only on the base of tests of specimens annealed sufficiently long time can it is possible to obtain informations, which can be than reliaby extrapolated to the exploitation time of many years. Because of the limited space only some most important original scientific findings are mentioned here. The work of development of the methodology of ''Small punch testing'', which enables to determine the resistance of steel to creep deformation on very small specimens, is continued. It was found that an eventual prior deformation is without effect on test results. This finding is important, because it is difficult to avoid all plastic deformation in the machining of very small specimens with the diameter of 8 mm and thickness of 0.5 mm. On the fracture surface of Charpy toughness specimens of a structural steel broken in transition temperature range a plane shearing band was found as passage from ductile to brittle crack propagation. Considering the width of the band, the critical radius of the crack tip and the width of the plastic zone were deduced. Amendements were proposed for the standard fracture mechanics equations for the calculation of both critical fracture mechanics parameters. With a number of tests on two different structural steels it was established that the rise of transition temperature after strain ageing was due to the synergy of processes of plastic deformation and annealing, while the change of tensile properties was related only to strain hardening. With observations of thin metallic foils of two different strain aged steels in transmission electron microscope, elongated diffused spots were found in the (110) ferrite plane and explained as due to elastsic stresses generated by interplanar segregation of carbon atoms. The relaxation of internal elastic stresses related to this segregation in lattice planes rich in dislocations was calculated also. The increase of transition temperature was explained in terms of decrease of cleavage strength due to the interplanar segregation of carbon. The investigation is continued in a program aimed to determine experimentally the cleavage strength of components of microstructure found in the heat affected zone of weld of low carbon high strength structural steels. For the steel X20CrMoV 121, which is used for thermal power works for parts operating at high temperature, the evolution of microstructure was investigated at annealing the quenched steel at 800 degrees C. The coarsening kinetics of carbide particles and the change of chemical and phase composition of particles were determined.