Friction stir process (FSP) and post thermal exposure were carried out to a microstructure-thermostable cast Al–5Mg–0.5Mn alloy to examine the grain size dependence of tensile yield stress by using Hall–Petch relation, σ_y = σ₀ + k_yd^−1/2. The measurement of preferred crystalline orientations and grain boundaries characteristic is used to explain the difference of the Hall–Petch parameters. FSP produces fine grain structure and, for the obtained average grain size from 3.7 to 12.8 µm, near-random crystalline orientations and certain proportion of rigid coincident site lattice boundaries (CSLs) is detected. Two-step post thermal exposure leads to a quick development of rotated ⟨001⟩ type crystalline orientations and the obtained coarse grains with average size from 229 to 508 µm are also commonly surrounded by general high angle boundaries. The calculation of Taylor factor and CSLs proportion reveals that stronger texture and grain boundary hardening effects exist in FSPed specimens than in thermal exposed specimens. These items are the major factors leading to larger σ₀ and k_y in FSPed specimens than in thermal exposed specimens.