In order to investigate the behavior of hydrogen isotope on the water-metal boundary, a series of deuterium permeation experiment from heavy water vessel through pure iron piping was performed as a function of temperature ranging 423~573 K at 15 MPa. During the experiment, the surface of iron piping was oxidized to magnetite at the heavy water boundary and then deuterium would generate by Schikorr reaction. This deuterium could be detected by mass spectrometer, which monitored the inside gases of the piping under vacuum. The result showed clearly that more than 85 % of the deuterium permeated through the metal piping and detected as deuterium gas (D2) under vacuum. The D2 permeation rate reached some stabilized value as a function of temperature.