Abstract: The effects of inhomogeneity of α phase and α' phase on strain concentration of heat affected zone of TA15 titanium alloy electron beam weld joint were investigated through instrumented nano-indentation test and tensile test of small specimens combined with the microstructure-based finite element method. The results showed that the yield strength and the hardening exponent of α' phase were higher than α phase. The α' phase was the "harder" microstructure, while the phase is the "softer" microstructure. Two phases had different mechanical properties, so the microstructures of heat affected zone were inhomogeneous. The inhomogeneity of microstructure resulted in the inhomogeneity of stress and strain. The α' phase had higher stress while the α phase had higher strain, which caused the incongruity of plastic deformation and eventually led to the plastic strain concentration bands along α phase.