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Extensive finite element analyses of cracked pipes with different crack sizes and orientations have been conducted to investigate effects of creep properties of materials on the unified creep constraint parameter Ac. The results show that the constraint parameter Ac is independent on Norton's coefficient A, and it is only affected by the creep exponent n of materials. For a given crack size, with increasing n, Ac decreases and constraint level increases. The Ac of lower constraint cracks is more sensitive to n. The unified correlation equations between Ac and n have been obtained for cracked pipes with a wide range of crack sizes and constraint levels. They may be used to estimate the constraint parameter Ac at different positions along the crack fronts in cracked pipes made of materials with different n values. The two-parameter C*-Ac approach for assessing creep life of cracked pipes has also been discussed. [ABSTRACT FROM AUTHOR]