The creep fatigue crack growth behaviour of P91 steel in as-received and ex-service material conditions has been examined. The crack growth data was characterized using fracture mechanics parameters ΔK and C*. The results showed that at high frequency (> 0.01 Hz), the CFCG behaviour tend to that of high cycle fatigue crack growth and
is best correlated with the ΔK parameter whereas at lower frequencies, creep mechanisms have been found to dominant and best correlated with the C* parameter. The correlation between crack growth rate and C* parameter,