A delayed crack propagation test using a thin sheet specimen of a single-crystalline Fe-3wt%Si alloy under constant load in a hydrogen environment was performed. A rectangular specimen (90 × 30 mm) was prepared from a sheet of material (thickness = 0.18 mm) with a (110) surface. A through-notch was introduced along the [1 ̅10] direction at the center of the specimen, and a fatigue pre-crack was then introduced on the (001) plane from the notch root (total crack length was 1.18 mm). An electro-hydraulic testing machine was used to conduct the test. A sustained load of 250 MPa was applied to the specimen in a chamber contained hydrogen atmosphere (580 kPa) at room temperature. After the test, topologies of the fracture surface and the side surface of the specimen were observed by field emission scanning electron microscopy (FE-SEM). Dislocation structure and plastic strain distribution beneath the fracture surface were investigated by electron channeling contrast imaging (ECCI) and electron backscattering diffraction (EBSD), respectively. The crack propagation was discontinuously and left striation pattern on the fracture surface. Extensive plastic deformation was observed to accompany crack propagation. The crack tip plastic deformation associated with hydrogen effect during the crack propagation leaves three adjacent regions beneath the fracture surface: (Region A) the region immediately beneath the fracture surface has an extremely high plastic strain, (Region B) the region adjacent to Region A has high dislocation density and high plastic strain, and its width is constant despite crack length increases, (Region C) the region away from the fracture surface has low dislocation density and low plastic strain, and its width linearly increases with crack length. These findings reveal the effects of plastic deformation and hydrogen-dislocations interaction around the crack tip on the rate-limiting process of hydrogen-induced delayed crack propagation in thin specimens.