The low cycle fatigue behavior of magnesium (Mg) AZ31B was observed at room temperature, in which its extrusion process led to differences in tensile and compressive stresses, with an increase in the grain structure and mechanical properties. The extrusion process results showed changes in the microstructure due to cyclic load-deformation at the longitudinal section with grain direction and shape. Furthermore, Mg AZ31B also showed transitional behavior from cyclic softening to hardening when the strain amplitude was increased. At a strain amplitude of 0.006 – 0.01 mm/mm, the Bauschinger phenomenon was observed. The effect factor was calculated by the yield stress and strain at compression stress. Furthermore, precipitation or local failure of the second phase was the main factor that caused the Bauschinger phenomenon. The fatigue fracture characteristics based on deformation due to cyclic loading include precipitation, fatigue striation, dimples, micro-cracks, and beach mark fatigue. Therefore, the correlation of the total failure cycle with plastic and the elastic strain was obtained as an equation to predict the lifespan of Mg AZ31B.