Magnesium has the lowest density among the engineering metallic materials and significant weight reduction can be achieved if magnesium alloys are used in automobiles, trains, airplanes, etc. However, they have some practical limitations in mechanical properties and corrosion resistance compared with other metallic materials. Generally, grain refinement is an effective way for enhancing the strength of metallic materials, and it has been reported that the grain refinement is very effective to strengthen the magnesium alloys. In this study, the influence of grain size on the tensile properties of three magnesium alloys has been investigated using specimens having grain sizes ranging from 100 nm to 100 µm. The grain size was changed by heat treatment after HPT processing, and the tensile tests have been made at an initial strain rate of 1.0×10−4 s−1. The grain refinement is effective for strengthening in the coarse-grained samples in which grain size is larger than 3 µm, but its effect decreases significantly in the fine-grained samples in which grain size is below 3 µm. This difference in effect of grain refinement has been discussed in relation to deformation mechanism.