Hydrogen-assisted ductility loss and related fracture morphologies were comprehensively discussed in consideration of hydrogen distribution in a specimen with external and internal hydrogen by using 300-series austenitic stainless steels (Types 304, 316L), low-alloy Cr-Mo steel (JIS-SCM435) and low-carbon steel (JIS-SGP). The external hydrogen represents a non-charged specimen tested in high-pressure gaseous hydrogen. The internal hydrogen represents a hydrogen-charged specimen tested in air or inert gas. Fracture morphologies obtained by slow-strain-rate tensile (SSRT) tests of the materials with external hydrogen (External-H) or internal hydrogen (Internal-H) could be explained via hydrogen-induced successive crack growth (HISCG) for External-H of Type 304 and JIS-SCM435, ordinary void formation for External-H of Type 316L and Internal-H of JIS-SCM435, small-sized void formation related to void sheet for Internal-H of Type 316L, large-sized void formation for Internal-H of JIS-SGP. Although various fracture morphologies were produced by external or internal hydrogen, these morphologies were inferred to be attributed to localized slip deformations by hydrogen. Also, substitutability between internal hydrogen testing and external hydrogen testing was validated according to the above fracture mechanisms.