The indicators of the brittleness and viscosity of metals calculated from their mechanical properties are considered with allowance for the stress state proceeding from the results of tensile testing of cylindrical smooth and notched samples of perlite 16KhSN, martensitic-aging (maraging) 03Kh11N10M2T, and austenitic 10Kh11N23T3MR steels. Tests were carried out on a UEM-10TM tensile strength testing machine, deformation diagrams developed on a scale of ~50:1 at a deformation rate of 5 mm/min. The sample size before and after testing were measured using a micrometer and an ISA-2 comparator with an accuracy of ±10–3 mm. Pendulum impact bending tests were carried out on a MK15 with the same cylindrical notched samples used to plot the plasticity and viscosity diagrams depending on the Bridgman stress state stiffness index. The new indicators of the brittleness λ = εk/η and viscosity η = (Sk/σb) – 1 (where εk = ln(1/(1 – ψk)) is the true limit plasticity) are proposed proceeding from the testing data. The special feature of the brittleness index λ is growth of the index with increase in the metal strength, e.g., due to pre-deformation or strengthening heat treatment procedures. However, a decrease in the groove radius on the samples, i.e., an increase in the Bridgman stress state stiffness, has almost no effect on the brittleness value λ, but is accompanied by a correlation decrease in the values of the viscosity indices η and the ultimate ductility εk of steels. The curves of the temperature dependences of the mechanical properties of steels 16KhSN and 03Kh11N10M2T show that anomalies in the brittleness indices λ observed at elevated test temperatures can be attributed to the structural transformations like increase in the grain size of 16KhSN steel or in the amount of the residual austenite in 03Kh11N10M2T steel due to reverse martensitic transformation. In this case, the temperature dependences of the viscosity η and brittleness λindicators change in the opposite way.