The temperature-dependent mechanical property of a forged Ti-6Al-4V (bimodal structure; α and α+ß phases) was investigated. Tensile test was performed at a wide range of temperatures between 77K to 550K. Fatigue test was also performed with corner notched specimens at the temperature range between room temperature and 550K. The temperature dependency of both the yield stress and activation volume was established from the tensile tests. The strong dependency of yield stress and activation volume was observed with respective to the temperature. At lower temperatures, from 77K to 250K, there was a decrease in yield stress with the increase in temperature, whereas abnormality in the yield stress was observed approximately at 275K, and the yield stress was decreased again with the increase in temperature at 325K. Activation volume was increased with increasing the temperature. The activation volume also shows abnormality at the temperatures where the abnormal trend was observed from yield stress. The reasons for the change in yield stress and activation volume with temperature were studied in detail.