The effects of the main process variables on the stir zone microstructure in friction stir welds were investigated for Ti-6Al-4V. Welds were produced by employing varying welding speeds under a constant rotation speed or different rotation speeds at a constant welding speed. The stir zone microstructure was examined by optical microscopy and transmission electron microscopy. It was found that the stir zone microstructure was determined by the parameters controlling temperature and deformation history during the friction stir welding. A bimodal microstructure characterized by primary alpha and transformed beta with lamellar alpha + beta or a full lamellar microstructure composed of basket-weave alpha + beta lamellae could be developed in the stir zone. The microstructural evolution mechanism in the stir zone was discussed.