Selective laser melting (SLM) is regarded as one of the most attractive metal additive manufacturing (AM) techniques, which has offered a promising solution to fabricating unique members and can be used for a wide range of applications. However, the SLM is often associated with fast cooling rate and a resultant large thermal gradient; which may lead to a large amount of plastic strains and coexistent residual stresses, and sometimes even result in cracking and failure during the fabrication. These plastic strains and residual stresses play crucial roles in determining the mechanical performance of the produced structural members. Plastic strains cannot be directly measured but can be characterized indirectly by hardness values. Thus, it is essential to study the distributions and magnitudes of both residual stresses and hardness in SLM-fabricated members. In the present study, comprehensive experimental studies on micro-hardness, material properties and residual stresses in SLM-fabricated H13 high strength steel hollow sections were conducted. Non-destructive neutron diffraction was utilized to measure residual stresses and their distributions across the wall thickness and over the cross-sections. The effects of both scanning patterns and cross-section geometries on the residual stresses and hardness were also studied. Linear relationships between the hardness values and the material strengths were established for the additively manufactured H13 steel. The Vickers hardness value was found to be highly correlated to the ultimate strength, while it was correlated to the 0.2% tensile proof stress with a small deviation.