“Environmental cell” microscopy was applied for surveying gas reaction of hydrides in magnesium base alloys, which are candidates for hydrogen storage materials in advanced hydrogen energy systems. In order to clarify the mechanism of hydrogenation process, in-situ experiment has been carried out by using a 200 kV transmission electron microscope (TEM) equipped with a newly developed environmental cell, which is capable to 0.1 MPa in the temperature range between R.T. and 200°C. When hydrogen gas reacted with magnesium powders, straightening of surface steps (60~70 nm in height) was observed, indicating that volume expansion occurred. In addition, the formation of MgH2 was indicated in selected-area-diffraction patterns (SADP). The precise study on this in-situ experiment, as well as its improvement, will be continued, with using transparent films.