Abstract: In the paper, based on the grain size observation and statistics analysis on TIG welded joints of TC4 titanium alloy, the gradient features of the gain sizes in welded joints with different heat input are studied experimentally for characterizing the microstructure heterogeneity by introducing a conception of grain gradient eigenvalue.The results show that the grain size in the TIG welded joint nonlinearly changes from the center of weld metal after the heat affected zone to the base metal, and the size changing rate(grain gradient) has a maximum value in the boundary between the columnar zone(CZ) and coarse grain zone(CGZ).The grain size gradient maximum value proposed as the grain gradient eigenvalue is inversely proportional to heat input in TIG welding and its position is independent with the heat input.The concept of the gain gradient eigenvalue can represent the microstructure heterogeneity in the welded zone and heat affected zone, and is significantly important for evaluating the service properties of titanium alloy welded joint.