Analyses based on the Hall-Petch relation and tensile loading-unloading tests were made to clarify the deformation mechanisms in pure Mg and AZ31 alloy polycrystals. The Hall-Petch slope and the frictional stress of both specimens decrease with increasing temperature, especially at higher than 423K. The fraction of the activated non-basal slip system is about 40% of all at 293K in pure Mg. The elastic recovery strain ratio epsilon(er)/epsilon(t), of pure Mg and AZ31 alloys decreases with an increase of the plastic strain epsilon(p) at the range of 293 to 523K, becoming almost constant at the strain epsilon(p) exceeding 1.5%. In consequence of decreasing with an increase of temperature, the ratio epsilon(er)/epsilon(t) at 523K is below half of that for 293K. The present results suggest that the non-basal slips activated in the plastic deformation at 293K, which cause to interfere with the back motion of piled-up dislocations on basal slip planes.