Titanium–6 aluminum–4 vanadium is isothermally forged at 1,023–1,223 K (750°C–950°C) at 0.05, 0.1, and 1.0/s using a 1,500-ton forging press simulator. The temperature increase of a workpiece (100 mm in diameter, 50 mm in height) during forging is measured by inserting thermocouples into the workpiece, and this increase is predicted using finite element analysis (FEA). The physical properties necessary for the FEA are determined using the same material as the forged one in order to improve the prediction accuracy. The true stress–true strain curves and heat transfer coefficient between the workpiece and dies are inversely obtained. The temperature change predicted by FEA using those parameters and curves agreed well with the experimental results. In the FEA, the inelastic heat fraction, which is a conversion factor for plastic deformation energy to heat energy, did not depend on the strain rate, but it remained the same at strain rates of 0.05–1.0/s. This result suggests that the large heat capacity of the workpiece in this study suppressed the effect of heat transfer between the workpiece and dies/air on the temperature of the workpiece. It is also shown that the prediction accuracy of the workpiece’s temperature could be degraded unless the appropriate specific heat value is used, especially near the β transus temperature, which can be affected by the composition of light elements.