This research aims to give the real data about affix errors in one short story. It applies qualitative method and library research. The writer also collected the theories of affix and error analysis for referents, read the short story, then tried to identify and analyzed every affix errors. In this finding, there are inflectional and derivational suffix errors. Inflectional suffix errors in the past –ed, past participle –ed, -s for the third singular person, and plural noun. And derivational suffix errors in using –ly, -ist, -y, ent-, and ed-. Those errors found in a short story written by Istiqomah, I Must Be a Man?