The role of the electroplated nickel layer on hydrogen permeation through AISI 4340 steel was investigated by a electrochemical hydrogen permeation test. The permeation test, composed of three steps, was conducted to measure the hydrogen diffusivity and surface hydrogen concentration. A constant current of 20 mAcm^-2 and a constant potential of -100 mV vs. Ag/AgCl electrode were applied to the hydrogen entry and exit cells, respectively. The thickness of the electroplated nickel layer on AISI 4340 steel increased in a linear fashion with an increase in electroplating time. The nickel coated layer contributed to a decrease in the hydrogen permeability of nickel coated AISI 4340 steel specimens. This is due to the fact that the surface hydrogen concentration and hydrogen diffusivity in nickel coated layer were lower than those of AISI 4340 steel substrate. Especially, low hydrogen diffusivity decreased significantly with hydrogen permeability. The critical effective hydrogen diffusivity for barrier of nickel electroplated AISI 4340 steel specimens was higher than the hydrogen diffusivity of AISI 4340 steel specimen. It is proposed then that the thin nickel layer on AISI 4340 steel acts as a barrier for hydrogen permeation through AISI 4340 steel.