The effect of deformation temperature (800-1000 degrees C) - strain accumulation (number of rolling passes: 3 divided by 5) - cooling rate (8,6 - 25 degrees C/s) parameters on the microstructure and mechanical properties of C-Ni-Nb and C-Ni-V steels after High Temperature Thermomechanical Processing (HTMP) are presented. The experiment planning method was used for the simulation and examination of the above mentioned parameters effect on the mechanical properties. The regression equations and diagrams describing the qualitative and quantitative effects of HTMP parameters on the mechanical properties of the steels are derived from the experimental results. The Ni-Nb-bearing steel with the lower bainite structure that is a result of the HTMP with the maximum number of passes, the highest deformation temperature and cooling rate shows the highest strength (YS=987 MPa) in combination with a high impact strength (W=154 J). It is shown that the Ni-Nb-bearing steel with the fine lower bainite structure that results from the transformation of the hot deformed fragmented substructure of austenite after water cooling has the best resistance to brittle fracture. The Nb-bearing steel has a higher strength compared to the V-bearing one, although it shows a lower impact strength in the negative testing temperature range.