For the further improvement of the press formability of steel sheets, it is important to clarify the relationship between macro mechanical properties and microstructure under multi-axial deformation state. The objective of this work is to develop the experimental system of in-situ observation and analysis for biaxial tensile deformation using electron back scatter diffraction patterns (EBSD) with scanning electron microscope (SEM). The appropriate shape of cruciform specimen for the system was examined first by using finite element analysis, and the biaxial tensile test system in vacuum SEM chamber was developed. In-situ observation of microstructure during equibiaxial tensile deformation was then conducted using the developed system and the proposed cruciform specimen. The material used in this study was an interstitial-free steel. It was validated by the comparison with the results obtained by the Marciniak type macro test that the developed system realized equibiaxial tensile deformation. Finally, some information obtained from SEM and EBSD analysis was illustrated. It was found for example that the grains with {001} plane orientations deformed easily and might cause the surface roughness.