Real-time observation of InP nanowire formation was performed using grazing incidence small angle x-ray scattering (GISAXS). Prior to the nanowire growth gold colloidal particles were spread on the substrate as the catalyst and annealed at 500 °C. Changes of GISAXS images were clearly observed after annealing, suggesting the formation of molten metal droplets, which were used for nanowire growth. After staring the growth little change except for the increase of GISAXS intensity was observed, suggesting that x-ray scattering from nanowires overlapped with that from the catalysts. From the GISAXS images calculated using a sphere and cylinder model, scattering from the nanowires shows the streak along the qydirection, and that from droplets shows an increase of intensity around the specular reflection, which qualitatively explains the measured GISAXS image after the growth. © 2007 IOP Publishing Ltd.