In order to make clear the crystallographic interpretation of each factor composing the Hall-Petch relational equation, experiments were performed by using two kinds of polycrystalline pure aluminum, which were annealed at 573 K and followed by quenching from 773 K. The In-Situ observations of the appearance or morphological change of slip bands were continuously carried out in the surface grains from elastic strain range to yielding. The main results obtained indicate that the Hall-Petch rule is proper for slip deformation in a polycrystalline aluminum which consists of the heterogeneous grain sizes. The Hall-Petch slope decreases with increasing of the number of active slip planes in the slipped surface grains just after yielding. Moreover, the friction stress after yielding shows a lower value than the yield stress of the single crystal, but the quench-harding increment in a polycrystalline aluminum is consistent with that of the single crystal.