In this study, uni-axial tensile tests of magnesium alloy sheet with thickness of 1.0mm were performed to establish stress-strain curve equation for magnesium alloy sheet (AZ31B) at elevated temperature of 100oC - 250oC. Stress-strain curve equations are established for magnesium alloy sheet at elevated temperatures describing both work-hardening and softening stage of this alloy sheet. Least square fitting method is used to determine material parameters. The parameters of the fitting curves are utilized to determine those parameters as functions of temperature using polynomial models. Proposed stress-strain curve models are used to calculate and predict the stress-strain curves at elevated temperatures. The new flow stress curves are in good agreement with experimental data and shown better accuracy than that of previously developed equations. Proposed hardening and softening model can be implemented to generate flow stress-strain curves for all high temperature materials e.g. Ti- alloys and Ni-based super-alloys and applied in FEM simulation.