The creep crack growth behavior in the base metal and a weld joint of X20CrMo 12 1 steel has been investigated under two-step loading at 545°C using 1/2T CT specimens. The crack growth process has been simulated using finite element analyses to examine the transient crack tip field. The results are compared with those under constant loading. In step-up loading, crack growth accelerated greatly after load change due to the effect of prior creep damage and reduced stress relaxation. In step-down loading, further creep-damage at the crack tip nearly ceased after load change due to a sharp drop in crack tip stress, leading to crack arrest for a substantial period of time. Crack growth eventually resumed as the crack tip stress increased by redistribution of the creep strain. Crack growth rates are found to correlate well with C*(t) for both base metal and weld specimens.