Dynamic change in distribution of strain generated in Si under a pad electrode was measured during ultrasonic ball bonding by using newly developed Si strain sensor. The sensor was designed to be able to determine strains in the directions normal and parallel to the surface. Bonding of Cu and Au was measured. It was clearly observed that the position of the largest compressive strain moved from the center of the ball to the periphery according to the progress of bonding under the application of the ultrasonic. Bonding of Cu was found to generate larger strain than bonding of Au. Bonding of Cu was also found to induce under-pad damage. SEM observation of bonded balls suggested that the under-pad damage appeared at the positon where the edge of the capillary existed and at the time when the large force of ultrasonic vibration was applied.