In order to understand the microstructure changes during welding processes , kinetic information about the phase transformation is essential. In our research group, in-situ observation systems consisting of undulator beam and imaging plate and two dimensional pixel detector have recently been used. These make it possible that phase transformation can be identified in real-time under the condition of directional-oriented solidification . In the present work, a combination of analyzing method: the in-situ observation system by X-ray diffraction technique using intense synchrotron radiation, and morphological observation by high-temperature laser scanning confocal microscopy is suggested to analyze the phase transformation during the welding process. Using the results acquired by these analysis methods, phase evolution of hypereutectoid carbon steel, during fusion welding was analyzed. The primary phase was directly identified as an austenite phase. Precipitation of pearlite phase was observed followed by the martensitic transformation.