The behavior of iron hydride was studied by in situ X-ray observations at 6GPa up to 700°C. When the iron-hydrogen system was heated, the dissolution of hydrogen into iron occurred, and bcc iron changed into iron hydride with a dhcp structure passing through a hcp structure, and then transformed into an fcc structure. The unit cell volumes per iron atom of these phases are much larger than those of bcc and hcp iron, which suggests that the iron hydride is formed. It was also found that the volume of the dhcp phase decreases with increasing temperature above 250°C, which indicates that the hydrogen concentration decreases at high temperatures.