The structural changes of the wrought magnesium alloy AZ31B in semisolid state were clarified using optical microscopy and hot-stage microscopy. The influence of heat treatment variables was assessed. Compression tests covering a range from room temperature to 673 K were carried out for mechanical property assessment; flow stress and breaking strain were determined. The following are the results: (1) The grain growth of the hot-extruded AZ31B without preprocessing sensitively reacted at temperature and retention time. (2) The hot-extruded AZ31B with 30 % preprocessing showed an almost perfectly spheroidized structure in a semisolid state under certain conditions. (3) Heating velocity markedly affected the spheroidizing rate of grains. (4) From the direct observation of the hot-extruded alloy AZ31B by hot-stage microscopy, spheroidization was observed in some crystal populations. (5) Spheroided materials in the semisolid temperature range had a lower flow stress and a larger breaking strain than nonspheroidized materials. These results indicate the possibility of manufacturing wrought magnesium alloy by cold working.