This paper describes the application of the reference stress approach and probabilistic methods to the determination of creep crack growth based on the time-dependent fracture mechanics parameter, C(t), where t is time. This parameter is defined as the simple sum of a transient component, C(t → 0), which is applicable to short times and a steady-state component, C*. The reference stress approach enables a relatively simple expression for C* to be derived. A scheme is developed that optimizes the fit of the reference stress approach to published computed solutions for Jp, the fully plastic component of the J-integral. The optimization scheme involves the derivation of an engineering parameter, V. An expression for C* is readily derived from an expression for Jp by invoking the creep-plastic analogy. Values of V are derived from the analysis of 189 sets of computed solutions. These values are statistically analyzed and used to derive a distribution function describing the uncertainty in V. This function is used together with distribution functions for other random variables (such as the creep strain rate coefficient and crack growth law coefficient) in example probabilistic analyses of flaws in welded internally pressurized pipes operating in the creep regime. Probability sensitivity factors are generated as part of the probabilistic analyses.