TC4 titanium alloy plate was jointed by friction stir welding in air and forced cooling condition separately, and microstructure transformation characteristics in different zones of the joint were investigated by OM and TEM. The result shows that the joint consists of stir zone, thermo-mechanically affected zone and base material, and they have different microstructure characteristics as a result of different thermal cycle and plastic deformation in different zones. Base material annealed after heat rolling is characterized by primary α and β phase. Stir zone is fully lamellar α+β structure, which is mainly controlled by phase transformation from β monophase to lamellar α+β phase in air. Thermo-mechanically affected zone exhibits bimodal microstructure of equiaxed α grain and α+β lamellar microstructure. Its transformation characteristic is dynamic recrystallization and phase transformation. Stir zone is characterized by acicular martensite in forced cooling condition, which is mainly controlled by martensite transformation