The palea and lemma (hull) are grass‐specific organs, and determine grain size and quality. In the study, AH2 encodes a MYB domain protein, and functions in the development of hull and grain. Mutation of AH2 produces smaller grains and alters grain quality including decreased amylose content and gel consistency, and increased protein content. Meantime, part of the hull lost the outer silicified cells, and induces a transformation of the outer rough epidermis to inner smooth epidermis cells, and the body of the palea was reduced in the ah2 muant. We confirmed the function of AH2 by complementation, CRISPR‐Cas9, and cytological and molecular tests. Additionally, AH2, as a repressor, repress transcription of the downstream genes. Our results revealed that AH2 plays an important role in the determination of hull epidermis development, palea identity, and grain size.