Abstract: A heuristic parameter estimation method for fault diagnosis is presented in this paper.Through applying knowledge to diagnosis process,the performance of traditional parameter estimation method is improved in the following two aspects:(1)The calculation labor for estimating the model parameters is reduced since the parameters estimated are reasonably limited to a small set.(2)The fault localization is more accurate as the equations p=f^-1(θ) are solved under the guidance of the physical meanings(expressed as knowledge) of the system.At the last of the paper a simple illustrative example is given.