In this work, we investigated the effect of Sb on the molecular beam epitaxy (MBE) growth of GaN and its optical properties. One monolayer Sb predeposition before GaN growth and different Sb beam equivalent pressures applied throughout the GaN growth were used to study the effect of Sb on GaN growth by ammonia gas-source MBE. The presence of Sb remarkably enhanced two-dimensional growth as evidenced by in situ reflected high energy electron diffraction (RHEED). RHEED patterns became streaky much more rapidly when GaN was grown in the presence of Sb than that without Sb, indicating that Sb can act as an effective surfactant to smooth the growth front of GaN and enhance the layer-by-layer growth mode for the MBE growth of GaN. The full width at half maximum of the (0004) x-ray diffraction rocking curve measured from the GaN epilayer grown with one monolayer Sb predeposited as surfactant was as narrow as 70 arcsec. In the photoluminescence measurement, besides the characteristic near band edge excitonic emissions, new transitions related to Sb-isovalent traps were observed from GaN samples grown with Sb, whose zero phonon line was located at 3.27 eV with phonon replicas on the lower energy side. Intensities of the emissions related to Sb-isovalent traps increased with Sb partial pressures applied during GaN growth.