AZ31B magnesium extruded slabs prepared from LFEC were rolled at fairly lower temperature at 3, 6 and 16 m/min rolling speeds into 1 mm thickness. The results indicate that the microstructures achieved by rolling at low temperature or at low rolling speed are composed of many prismatic regions divided by shear strips due to pile-up of twin crystals; the prismatic regions increase at elevated rolling temperature or at high rolling speed, and finally all are composed of equiaxed crystals without twin crystals due to dynamic recrystallization. After optimizing control of rolling process, excellent mechanical properties would be acquired. The mechanical properties of AZ31B sheet are σb=350 MPa, σ0.2=300 MPa, and δ=12.0% when rolled at 6 m/min. At the same time, the difference of mechanical properties between transverse and longitudinal direction reduced markedly.