We report a study on surface morphology and optical properties of GaAsN epilayers grown on GaAs (1 0 0) vicinal substrates by metalorganic vapor-phase epitaxy (MOVPE), using dimethylhydrazine as the N source. The surface morphology was investigated by atomic force microscopy (AFM) as a function of N composition and growth temperature (T-g). At T-g = 530-550 degreesC, we observed a transition from a bunched step/terrace structure towards a surface ith indistinct step/terrace structure when increasing the N content. We attribute this evolution to a transition From step-bunching towards 2D-nucleation growth, related to decreased Ca surface-diffusion length. The optical properties were studied using room-temperature (RT) and low-temperature photoluminescence (PL), and photoreflectance (PR). Post-growth thermal annealing were carried out on GaAs-capped layers. A drastic increase of the RT PL intensity and a narrowing of the linewidth were observed under optimum annealing conditions. The influence of annealing temperature and duration were also examined.