**Abstract**

Previously, we have proposed creep deformation law estimated by non-dimensional time representation to predict creep fracture and remnant life. Furthermore, using steady state creep rate coupled with crack growth rate law based on *Q\** parameter, *QL\** parameter was derived and it was found to enable us to discriminate creep ductility and predict creep fracture life. In this study, a quantitative estimation and a prediction methods of mechanical performance on creep strength (MPCS) and creep fracture life of the creep ductile materials including a weld joint notched specimen was noticed and the following studies were conducted. 1) The similarity law of creep deformation, 2) *QL\**map, which discriminates creep ductility and predicts creep fracture life, 3) Derivation method of mechanical indicators, "Converted stress", and *ΔQL\**, which quantify MPCS, 4) Example of the quantitative estimation by these parameters using P91 steel and its weld joint notched specimens. From these results, the concept of the converted stress and the *ΔQL\**were found to enable us to conduct quantitative estimation of MPCS and prediction of creep fracture life, with the short experimental period, the small number of specimens, the reasonable accuracy and an economic efficiency, which is an engineering significance.

**Keywords:** *QL\**, MPCS, converted stress, creep ductility, weld joint, creep fracture life, *ΔQL\**
