At high temperatures typical for service conditions in fossil power plants, the creep fracture is dominated by the formation, growth and coalescence of cavities. Using high temperature pipe materials, P92 and P122, the characteristics of creep crack growth were analyzed in this study according to the cavities. The characteristics of cavities play a critical role in creep crack propagation and load line displacement. The effect of the load line displacement rate(dv/dt) and crack growth rate(da/dt) on the da/dt–Ct relation of creep crack growth was evaluated at different temperatures and Ki(initial stress intensity factor) values. The number of cavities increased with increasing temperature and Ki. The crack growth rate and load line displacement rate increased with the increase in the cavity numbers. The kind and distribution of these internal flaws were investigated by an intelligent phased array ultrasonic method and they were utilized in deriving the relationship with the creep crack growth rate, which will predict the creep characteristics of these materials.