Abstract: TIG welding experiments on 20 mm thick TC4 titanium alloy plate were carried out by two varying heat inputs,and the joints were investigated in terms of microstructure,mechanical properties and welding defects.Microstructural observations showed that martensite in welded zone is more dispersed than those in fusion zone and heat affected zone.in Heat affected zone has less quantity and small size of martensite.With the welding heat input increasing,the grain size of the joints grows and martensite becomes larger and more dispersed.High heat input leads to stomata defect and deteriorate the mechanical properties of weld joint seriously.To avoid the defects such as abnormal structures and cracks,the welding heat input should be strictly controlled to prevent the coarse grain in the weld joints.