Z-parameter method based on the Larson-Miller relationship is proposed to assess remaining life of a 1Cr5Mo steel. The non-linear master curve of stress a vs. Larson-Miller parameter P of the steel can be expressed as: P=29.608-5. 085logσ. A family of curves parallel to the master curve can be expressed as: P=29.608-Z-5.085logσ, where the value of Z represents the magnitude deviated from the master curve. A relationship between the value of Z-parameter and the level of spheroidization E (mi-crostructural degradation) can be expressed as: Z=0.203(E-1). The non-linear master curve of stress a vs. Larson-Miller parameter P of the steel can expressed as: P = 29.608- 5. 085logσ. A family of curves parallel to the master curve can be expressed as: P = 29.608-Z-5.085logσ, where the value of Z represents the magnitude deviated from the master curve. A relationship between the value of Z-parameter and the level of spheroidization E (mi-crostructural degradation) can be expressed as: Z = 0.203 (E-1).