PROBLEM TO BE SOLVED: To provide a method for determining fatigue level of a subject, a method for determining efficacy of a subject material against fatigue, a method for searching a material which can relieve, alleviate or prevent fatigue, and a reagent or kit for determining fatigue, with use of a biological specimen which can be easily prepared from a subject.SOLUTION: A method for determining fatigue level includes an analysis and/or comparison of expression level of an oxidized phospholipid and/or a metabolite thereof in a subject-derived specimen.