Time-resolved X-ray diffraction system was studied to characterize the unidirectional solidification and the phase transformation during welding. Experimental conditions including wavelength of X-ray, scattering angle, incident angle and time-resolution for analyzing the solidification process in welding were made clear. Furthermore, heating and solidification processes for weld metal of 13Cr–9Ni steel in TIG welding were in-situ analyzed by using the system. The primary phase was identified as delta-phase during solidification with halo pattern for liquid phase, followed by the peritectic reaction with a spot-like diffraction pattern for delta-phase and austenite phase.