The invention relates to a gene regulating the grain length of a plant seed and the use thereof. For lk3 locus identified by a detailed linkage analysis of the lk3 locus through large-scale segregating population, a base sequence analysis and gene prediction were carried out. Then, for a predicted Lk3 gene, its function was confirmed through transformation, thus the invention has been completed. The invention provides an Lk3 gene regulating the grain length of a plant seed. The invention also provides a method of transforming a plant comprising the step of performing transformation such that the function of the gene of the invention is reduced. According to the invention, a transformed plant in which the grain length is regulated can be obtained. Preferred examples of such a plant include rice.
