We investigated the suppression mechanism of strain-age-hardening in a ferritic carbon steel associated with hydrogen uptake. We considered hydrogen-related three factors suppressing the strain aging: 1) solution softening, for instance, arising from a reduction in Peierls potential of screw dislocations and a change in Young’s modulus, 2) suppression of dislocation-carbon/nitrogen interaction through hydrogen/carbon and nitrogen site competition, and 3) change in plastic strain evolution behavior by hydrogen-enhanced localized plasticity (HELP). According to the present experiments under in-situ hydrogen charging, it was concluded that the solution softening (factor1) and the site competition (factor2) by hydrogen did not significantly suppress the strain aging but the change in the pre-straining behavior (factor3) did.