In this study, to achieve constitutive equations of Ti-6Al-4V alloy sheet, uni-axial tensile tests of Ti-6Al-4V alloy sheet with thickness of 1.0mm were performed at elevated temperature of $400^{circ}C{sim}700^{circ}C$. Equations of stress-strain curves are established for Ti-6Al-4V alloy sheet at elevated temperatures describing both work-hardening and softening stage of Ti-6Al-4V alloy sheet. Least square fitting method is used to determine material parameters. The parameters of fitting curves are utilized to determine them as a function of temperature using polynomial models. New mathematical models are used to calculate and predict the stress-strain curves at elevated temperatures. The calculated flow stress curves are in good agreement with experimental data and shown better than that of previous equation.