The as-cast pure magnesium (Mg), with a purity of 99.99％, was hot-extruded at 300℃ to prepare a Mg bar with a diameter of 8 mm. The microstructure and mechanical properties of the sample before and after extrusion were investigated. The results show that the as-extruded microstructure is obviously refined with a large number of subgrains rather than equiaxed grains. A great number of (102) tensile twins can be observed significantly in the microstructure at the temperature. Mechanical properties including yield strength (YS) and ultimate tensile strength (UTS) increase greatly but uniform elongation (UE) decreases slightly as a result of work hardening.