Data of metal fatigue test is described with Weibull distribution which is asymmetrical. But general methods for outlier detection in metal fatigue tests are based on symmetrically statistical distribution. The difference between Weibull distribution and symmetrically statistical distribution is often found in the areas of small life and high life where outlier appears. A method of outlier detection is derived based on Weibull distribution in this paper. And some reasonable measures are used in the derivation. By comparison, the method in this paper is more reliable than those based on asymmetrical distribution.