We have studied the ion-beam synthesis of GaSb nanocrystals in Si by high-fluence implantation of Sb and Ga ions followed by thermal annealing. RBS, TEM/TED, RS, and photoluminescence (PL) were employed to characterize the implanted layers. It was found that the nanocrystals size increases from 5 to 60 nm in the samples annealed at 900 °C up to 20–90 nm in the samples annealed at 1100 °C. An existence of significant mechanical stresses within implanted layers has been detected. The stress values have been calculated from the shift of the Si first order Raman band. For the samples annealed at 900 °C a broad band in the spectral region of about 0.75–1.05 eV is detected in the PL spectra. The nature of this PL band is discussed.