Abstract: The θ-function method is used to calculate pressured parts inside boiler as the fourth stage on the lifetime evaluation and has the advantages of high precision and long extrapolation over Larson-Miller extrapolation formula.In order to achieve lifetime on line monitoring of pressured parts inside boiler,the lifetime prediction programs are designed and programmed using VC based on θ-function method,and the data fitting of θ-function is realized in this paper.Then the analysis on creep lifetime loss is conducted from the data fitting results and the calculating processes of creep lifetime loss are given.Finally,according to the operating data,the online evaluation is undertaken with θ-function method for creep lifetime loss analysis of a high-temperature superheater outlet header using metal P91 in a power plant.The θ-function method is used to evaluate pressured part lifetime loss in this power plant based on the practical operation data.