Abstract The indentation test is often used to determine the mechanical properties of materials, and its results are typically compared with uniaxial test results. However, the relationship between the indentation and uniaxial tests is unclear. Previously, the limit at which the indentation and uniaxial tests can be related has been determined by finite element simulation and termed the critical strain of indentation. In the critical strain concept, the high strain of the stress–strain curve has a negligible effect on the indentation load–displacement curve. To date, the effect of critical strain has not been addressed in indentation experiments. In this study, the serration behavior of an Al-Zn-Mg alloy during natural aging was investigated using uniaxial tensile and indentation tests in order to clarify (i) the relationship between these tests and (ii) the critical strain of indentation. Similar to uniaxial tests, the serration behavior of indentation was affected by the strain rate effect. However, it was found that the serration behavior observed at high strain in the uniaxial test could not be observed in the indentation test, as indicated by the concept of critical strain. With aging, the serrations of the indentation disappeared; however, they were observed in the uniaxial tests. It was confirmed that the migration of Mg atoms affected the serration behavior of indentation. The critical strain was experimentally found by comparing uniaxial test and indentation test results.