We report a first-principles density functional theory study on the role of grain boundary and dislocation loop in H blistering in W. At low temperature, the Σ3(111) tilt grain boundary can trap up to six H atoms per (1×1) unit in (111) plane before significant sliding occurs. This amount of H weakens greatly the cohesion across the boundary. At room temperature, when only three H can be trapped, this effect can be still significant. A dislocation loop in (100) plane can trap four H per (1×1) unit even at room temperature, whose detrimental effect is strong enough to break the crystal. Our numerical results demonstrate unambiguously the grain boundaries and dislocation loops can serve as precursors of H blistering. In addition, we find no H2 molecules can be formed in either environment before fracture of W bonds starts, well explaining the H blistering in the absence of voids during non-damaging irradiation.