Refining a metal’s grain size can result in dramatic increases in strength, and the magnitude of this strengthening increment can be estimated using the Hall–Petch equation. Since the Hall–Petch equation was proposed, there have been many experimental studies supporting its applicability to pure metals and alloys. Several studies on bcc metals have shown that even when these materials twin during deformation, their yield stress is still related to the grain size through the HP equation. We have gathered the grain-size strengthening data from the Hall–Petch studies on bcc and nanocrystalline metals and used this aggregated data to calculate best estimates of these metal’s Hall–Petch parameters. We also use this aggregated data to re-evaluate the various models developed to physically support the Hall–Petch scaling.The aim of our present work is to investigate the effect of Hall-Petch relationship and see the variation in H-P parameters like 𝜎0 and 𝑘 on the substructure evolution in the grain interior and grain boundary region of some bcc and nanocrystalline metals. While the data are traditionally fitted to the inverse square-root dependence, they also fit well to other functions, both power law and non-power law. Through our analysis, we made an attempt to understand the behaviour of grain interior and grain boundary region which can guide us about the microstructure evolution during deformation in the material. It is important to note that for nanomaterials with grain sizes of several tens of nanometres this law, to a certain extent, is not observed, giving way to the so-called inverse Hall-Petch effect.Keywords: Hall–Petch relationship, Grain-size, Grain Boundary, Grain interior, BCC and Nano crystalline metals, Inverse Hall-Petch effect.