The present invention relates to an isolated major gene GS3 which regulates grain weight and grain length in the rice and the cloning of said gene. The DNA sequence of GS3 gene is as shown in SEQ ID NO. 1 and is 7883 bp in length. GS3 gene comprises 5 exons and encodes 232 amino acids. It is predicted based on bioinformatics analysis that said protein contains conserved domains including a PEBP-like domain, a transmembrane domain, a cysteine-rich domain of TNFR/NGFR and a VWFC domain. cDNA sequence of said gene is as shown in SEQ ID NO. 2. By sequence alignment between three large grain species and 3 small grain species of rice, it is revealed there is only one common single nucleotide mutation in a 7.9-kb region between the two different grain-length groups. Said nucleotide mutation is located at the second exon of the GS3 gene, in which a cysteine codon (TGC) in the small-grain group is mutated to a termination codon (TGA) in the large-grain group. This mutation causes a premature termination in the large-grain group, which leads to a 178-amino acids truncation (including part of the PEBP-like domain and all the other three conserved domains). The present invention also provides methods of producing transgenic plants comprising sequences disclosed herein.