The invention discloses a rice grain width gene GW5L and an application thereof, and provides an application of protein GW5L, a DNA molecule encoding the protein GW5L or a recombinant vector containing the DNA molecule encoding the protein GW5L in regulation of the grain shape of a plant. The gene GW5L is transferred into wild rice, and transgenic rice is obtained. Compared with recipient rice, the gene GW5L in the transgenic rice is overexpressed, the grain width of the rice is reduced substantially, and the grain length is increased significantly. A foundation is laid for cultivating transgenic plants with changed grain shapes.