The combined creep-fatigue loading tests and the fatigue tests with tensile strain-hold wave form were conducted on 2 1/4Cr-1Mo steel, and the creep damage, φc
, and the fatigue damage, φf
, which were accumulated till rupture were calculated using the linear life fraction damage rule. The creep loading conditions of the both tests were within the transglanular fracture region on the creep fracture mode map obtained from the creep rupture tests. The specimens in the combined creep-fatigue loading tests were ruptured by an accumulation of either φc
, and the values of φc
, and φf
at fracture were 1.3 and 1.0, respectively. This φc vs.
relation for 2 1/4Cr-1Mo steel showed the same trend as that for SUS304, SUS316 and 1Cr-Mo-V steels obtained from the combined creep-fatigue loading tests under the same creep damage mode condition, but the value of φc
was dependent on testing materials. The φc
-value seemed to be related with hardening or softening of materials due to fatigue loading.
The φc vs.
relation evaluated from the fatigue tests with tensile strain-hold wave form showed a different tendency to that evaluated from the combined creep-fatigue loading tests for 2 1/4Cr-1Mo steel. 1Cr-Mo-V steel showed the similar trend as 2 1/4Cr-1Mo steel, but the φc vs.
relations obtained from both tests was the same for SUS316 steel. The effect of testing procedure on the φc vs.
relation seemed to be connected with the sensitivity to oxidation of testing materials.