A new mathematic approach was proposed to evaluate the temper embrittlement from the step cooling test. The approach was composed of two parts. A nonlinear model, which acted as weight factor, was used to fit the variance at different test temperatures, and the maximum likelihood method was used to determine the curve parameters. In the ASME specification SA-542, there was no instruction to be provided on how to determine the transition temperature from the test results. This method is adopted as an additional supplementary requirement to assess the temper embrittlement of Chromium-Molybdenum steel.