Dynamic changes in distribution of mechanical strain generated during wire bonding in Si under and near the bonding pad were measured by using a piezoresistive linear array sensor. The sensor was designed to be able to determine strains in the directions normal and parallel to the surface. Bonding dynamics of Cu and Au balls were investigated. We can clearly observe the oscillating strain according to the application of 150 kHz ultrasonic vibration. It was also 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 vibration. Bonding of Cu was found to generate larger strain than bonding of Au. A large oscillating tensile strain generated at the periphery of Cu ball when ultrasonic amplitude is increased is found to cause fracture of Si. The largest residual strain is observed for Cu bonding at the location where the end of capillary tool was present during bonding.