Abstract: The quality of TiAl and 40Cr diffusion welding interface was inspected by ultrasonic C-scan method and the relationships between mean and standard deviation of amplitude of interface echo and shear strength were studied.The effects of weld temperatures and surface roughnesses of specimens on above-mentioned relationships were investigated and the relationship between the amplitude and length of air gap was discussed.It is found that at a given surface roughness, bonding strength increases with the decreasing of mean and standard deviation of amplitude.The relationships between mean and standard deviation of amplitude of interface echo and shear strength are influenced by surface roughness.When the length of air gap is shorter than 400 μm, the amplitude of interface echo is approximately in proportion to the length of air gap.