We investigated the heteroepitaxial growth of GaSb on Si(001) substrate. We found that high-quality GaSb films can be grown on Si substrate by introducing an AlSb initiation layer. We obtained a narrow X-ray diffraction rocking curve and over-1400-nm emission from GaSb/AlGaSb quantum wells by optimizing the growth temperature and thickness of the AlSb initiation layer. During the growth of GaSb, the AlSb initiation layer played two important roles for improving crystal quality; i.e., it acted as a surfactant and as a buffer layer preventing generation and propagation of dislocations.