Methods are disclosed to achieve an improvement in the characteristics and yield of cotton fibers. In one method, a cotton plant of the genus Gossypium in seed form or in growth stage is treated with a brassinosteroid. In another method, an ovule culture is prepared from a cotton plant of the genus Gossypium in a brassinosteroid-containing liquid medium. Cotton fibers with improved fiber characteristics are obtained from the cotton bolls of the treated plant or from the cultured ovules. Also disclosed are a method for inducing specific genes expression in a cotton plant to produce cotton fibers with improved fiber characteristics by treatment with a brassinosteroid, and a cotton plant produced by this method; as well as a novel gene capable of changing the degree of its expression found in the ovules of a cotton plant treated with a brassinosteroid, a gene capable of hybridizing with this novel gene, a recombinant plasmid containing this novel gene, a transformant containing this recombinant plasmid, and the like.