Abstract: A process of applying the response surface methodology (RSM) to the acquisition of the function in ultimate state, which is characterized with the quadratic polynomial response surface function, is briefly described. A new method is proposed to analyze the sensitivity of reliability via the function in ultimate state. An example is given to prove that the method proposed is available to the unknown function in ultimate state, with some formulae deduced. The function in ultimate state obtained by RSM, as a simple quadric polynomial, naturally includes the information on linear, quadratic and cross quadratic terms, thus making the sensitivity calculation of reliability easy to do and its accuracy very high. With the cracking due to thermal fatigue exemplified, the reliability without destabilized extension happening is calculated, as well as its sensitivity with different random variables.