This paper summarizes the achieved results of microstructure analysis and mechanical testing of long-term degradation of Mn-V steel (W. Nr. 1.1133, ČSN 41 3123) frequently used in small and middle boiler pressure systems operating under creep conditions. In these systems, non-exchangeable components often limit a safety of their service operation. Therefore in view of the monitoring, non-destructive methods like replica technique, hardness testing and defectoscopy, and probabilistic approach to residual life assessment seems to be the only way how to quantitatively determine a residual life of the boiler pressure system. Achieved results of as-exposed and as-secondary aged Mn-V steel confirmed that besides NDT controlling of sheet welds due to possible cracks, the replica technique (on available component surface) accompanied by hardness measurement on each segment of weldment should be sufficient for residual life assessment of thick-walled components of boiler pressure system operating under creep conditions. Additionally as a very rough estimate, a value of limit hardness of steel for safety operation is estimated at about 150 HV.