Tensile creep tests on 10CrMo910 heat resisting steel and 316 stainless steel were carried out for 10000 hours. On the basis of the experimental results, the isochronous stress-strain curves and time- dependent failure assessment curves (TDFAC) of these two steels were given. Finally, the corresponding formulae for long-term creep state, which could be utilized in the high temperature defects assessment, were established.