Isothermal forging die plays an increasing important role in plastic forming. During isothermal forging process, die bears high and long-time thermomechanical loading, which leads to short service life and poor reliability. Therefore, estimating isothermal forging die life using traditional method will engender great errors. In this paper, an average strain of fatigue process zone in danger region is introduced to estimate service lifetime of isothermal forging die. An upper die of a Ti-4Al-6V disc is analyzed to investigate the service life of die. The result shows that using average strain method can estimate service life of isothermal forging die more exactly.