Abstract: This study investigated the influences of prior-austenite grain size (PAGS) ranging 22~62 μm on crystallographic characteristics and hierarchical structures of bainite and martensite in a high-strength low alloy steel. Bainite dominates when PAGS ≤52 μm, whereas martensite prevails at PAGS of 62 μm due to elevated martensite start temperature. Utilizing electron backscatter diffraction analysis and prior austenite orientation reconstruction, crystallographic characteristics (variant selection and variant pairing) were analyzed and over 100 prior austenite grains were examined per specimen. With increasing PAGS, the average variant types total quantity in bainite specimens is 14.6, 14.1, and 18.3 types per prior-austenite grain, indicating weakened variant selection at larger PAGS. The smaller PAG tend to form fewer variant types due to insufficient autocatalytic nucleation. Variant pairing in bainite specimens shows decreasing the fraction of V1/V2 and V1/V4 pairs and increasing the fraction of V1/V8 pair with larger PAGS. In contrast, the martensitic specimen favours the formation of variants within the same CP group, particularly the self-accommodating V1/V2 pair. This is attributed to the enhanced austenite matrix strength and the reduction in transformation temperatures at finer PAGS, which promote transformation strain self-accommodation. Regarding hierarchical structures, for PAGS ≤52 μm, the number of packets, Bain grains, blocks and sub-blocks per prior austenite grain and their mean area decrease with finer PAGS mainly due to spatial constraints. A PAGS of 62 μm leads to packet coarsening, while other hierarchical structures undergo refinement. This study provides a statistical framework for crystallographic characteristics and hierarchical structure analysis in steels, offering insights into optimising bainitic and martensitic transformations through PAGS regulation.