In this paper, a prediction of C(t), a crack tip parameter for the transient creep condition, for the circumferential cracked pipe under combined mechanical and thermal stresses are presented. The estimation formulae for C(t)-integral of the cracked component operating under mechanical load alone have been provided for decades [1-6]. However, high temperature structures usually work under combined mechanical and thermal load. And the interactions between mechanical and thermal loads make the relaxation rate different from those produced under mechanical load alone. In this study, 3-dimensional finite element analyses are conducted to calculate the C(t)-integral under combined mechanical and thermal load. As a result, redistribution time for the crack under combined mechanical and thermal load is re-defined to quantify the C(t)-integral. The estimation of C(t)-integral using this proposed redistribution time agrees well with FE results.