The isothermal transformation kinetics of U-2Nb alloy was investigated. Based on metallographic quantitative measurements, the general kinetics was presented with the time-temperature-transformation (TTT) diagram. Results show that considerable discrepancies are identified in comparison with previous work on similar alloys. The differences are mainly attributed to two different transformation mechanisms, namely the monotectoid and the discontinuous precipitation reactions, which are operated at higher (550~647 °C) and lower (450~550 °C) temperatures, respectively. In addition, the growth rate of the acicular α precipitate in its lengthening direction, which proceeds through monotectoid reaction, was discussed. Modeling of the lengthening rate was carried out with the Zener-Hillert model supposing a diffusion-controlled mechanism.