Development of semi-dwarf crop varieties has significantly increased the world food production, which is referred as 18Green Revolution 19. The semi-dwarfing varieties had better lodging resistance, higher harvest index, and more efficient utilization of the environment. The green revolution genes have been isolated from rice (sd1) and wheat (Rht). The sd1 gene encodes a gibberellin acid (GA) 20 oxidase enzyme, which controls a step of biosynthetic pathway of gibberellin, and the Rht is a GA-insensitive gene which controls gibberellin signaling pathway. Two barley semi-dwarf genes sdw and denso have been widely used for barley improvement. The two genes originated from different sources but have been proved to be allelic. The sdw/denso gene was mapped to the long arm of chromosome 3H. Comparative analysis revealed that barley chromosome 3H is syntenic to the rice chromosome 1, to which the rice semi-dwarf gene sd1 was mapped. Further analysis showed that the sdw/denso gene is in the same syntenic region with the rice sd1 gene closely linked with a common RFLP marker R1545. This syntenic relationship was further confirmed through wheat and rice comparative analysis. Both barley sdw/denso and rice sd1 are GA-sensitive semi-dwarf genes. Thus, sdw/denso in barley is the most likely orthologue of sd1 in rice. The rice sd1 orthologous gene (Hv20ox2) was partial isolated from barley based on the conserved sequences between rice, wheat and maize. The barley gene has the same structure as its orthologous gene in rice with three exons and two introns. The barley and rice genes shared 88.3% sequence similarity and 89% amino acid identity. We are in the process to sequence various alleles of this gene from barley to identify the gene mutations for semi-dwarf phenotype and to develop molecular markers marker-assisted selection.