This study focuses on the role of pyramidal 〈c + a〉 dislocations in the grain refinement mechanism in the Ti-6Al-4V alloy with an initial {112¯0}<101¯0>rolling texture. A large number of pyramidal 〈c + a〉 dislocations were activated in the sample subjected to the severe shot peening process. Two important roles of pyramidal 〈c + a〉 dislocations were discovered. First, pyramidal 〈c + a〉 slip coordinates the large c-axis strain, thereby achieving generalized plastic flow, especially in nanograins. Second, the unique low-angle grain boundaries (LAGBs) with basal-pyramidal dislocation locks (prismatic 〈c〉 and prismatic 〈c + a〉 dislocations) were produced for the first time by pyramidal 〈c + a〉 interacting with basal 〈a〉 dislocations. This unique low-energy boundary greatly enhances the stability of the strain-induced grain boundary and dislocation density (~6.6 × 1015m − 2 in nanograins). The grain refinement process contains three types of subdivision modes: (I) dislocation walls with pyramidal 〈c + a〉 dislocations in coarse grains; (II) basal 〈a〉 intersecting with prismatic 〈a〉 dislocations in coarse grains; and (III) basal 〈a〉 intersecting with pyramidal 〈c + a〉 dislocations in coarse grains, ultrafine-grains and nanograins. The occurrence of slip modes depends on the initial texture and texture evolution during dynamic recrystallization. Besides, Hall-Petch breakdown at the nanoscale was found and is attributed to the decreasing critical resolved shear stress of pyramidal 〈c + a〉 slip at the nanoscale. This study provides a new approach for the design of stable nanostructured hexagonal close-packed metals by the unique LAGBs with basal-pyramidal dislocation locks. Display omitted