The combined effects of temperature and frequency on the crack growth rate and failure life were investigated using the compact tension specimens of 12Cr steel under creep and creep-fatigue conditions. The creep-fatigue tests were performed by introducing the hold time at the maximum load. At higher frequency where the crack growth rate near the initial stress intensity factor or reciprocal of failure life is proportional to the frequency, the cycle-dependent fatigue process is dominant. The Vickers hardness around the path of crack growth was obtained as a measure of damage during creep and creep-fatigue. Complex fracture surface or branched crack is attributed to the decrease in hardness for creep-fatigue interaction and is characterized by fractal analysis.