【Objective】Rice grain shape is a complex quantitative trait controlled by multiple genes, which affects rice yield, rice appearance and milling quality simultaneously. It is of great significance to explore the genes related to grain shape and elucidate its genetic basis. Previously, a gene, namely GS2.2, which controls rice grain size, was mapped on the chromosome 2, by using a set of near isogenic lines derived from the small-grain japonica rice variety Nipponbare (NPB) as the recurrent parent, and an indica rice variety Tedaxian(TDX) as the donor parent. 【Method】In this study, fine mapping of GS2.2 and identification of candidate functional genes were carried out. 【Result】 GS2.2 was fine mapped to the 160.6 kb interval between the makers of 2M-7-1 and 2M-9, by using a BC₄F₂ population with 2887 small grain size plants and a BC₄F₃ population with 70 large grain size plants. Bioinformatics analysis showed that the 160.6 kb chromosome interval encodes 18 open reading frames (termed as ORF1-18, respectively). Sequence analysis showed that there were five SNP differences in ORF18 between the large grain near isogenic line and Nipponbare. ORF18 encoded an auxin regulatory factor protein OsGRF4. The grain length of ORF18 knockout lines, whose ORF18 was knocked out in large grain near isogenic lines by using CRISPR/CAS9 gene editing technology, became smaller. The above results confirmed that ORF18 was a functional gene of GS2.2. Further identification of ORF18 near isogenic lines (ORF18-NIL) and gene editing knockout lines by in vitro inoculation and disease nursery inoculation showed that the rice blast resistance of ORF18-NIL increased, while that of gene editing knockout lines decreased, indicating that ORF18 also positively regulates rice blast resistance. 【Conclusion】The auxin regulator OsGRF4 found in this study simultaneously regulates rice grain shape and blast resistance, which provides an important gene for breeding rice variety with high yield in coordination with enhanced resistance.