The Hall-Petch relation was confirmed to be found between the flow stress and the grain size at the temperatures from 77K to 523K for pure Mg, and at 293K for AZ31 Mg alloy polycrystals. The Hall-Petch slopes k(y) are nearly independent of the strain up to 3%, however, the frictional stresses sigma(0) increase with increasing strain because of work hardening. Owing to solution strengthening, the values of k(y) and sigma(0) for 0.2% proof stress sigma(0.2) at 293K of AZ31 alloys, increase in comparison with those for pure Mg. Also, the values of k(y) and sigma(0) for sigma(0.2) of pure Mg decrease with increasing temperature, especially at higher than 423K. The reduction of k(y) values mainly contributes to an increase of the number of activated slip systems, that is basal or non-basal slip systems. Percentage of the activated non-basal slip is estimated to be about 40% of all at 293 K in pure Mg polycrystals, by considering both the sigma(0) values and CRSS of pure Mg single crystals. This means that the non-basal slips play an important role on the plastic deformation of pure Mg polycrystals at 293K.