Impact of bonding temperature on bonding dynamics in the ultrasonic bonding of Cu FAB (free air ball) is investigated by measuring dynamic strain with piezoresistive strain sensor. The strain sensor was composed of an array of pairs of n-and p-type strain gauges to determine the strains along two axes, the axis of ultrasonic motion and the axis perpendicular to the surface of the substrate. Au wire bonding was also investigated for comparison. The substrate temperature was changed from room temperature to 150°C. It is clearly observed that deformation of FAB is accelerated by elevating the substrate temperature. Most of the deformation of FAB is found to be completed within several milliseconds after the start of the bonding operation. For the case of Au FAB bonding, significant reduction of residual strain was observed at 150°C.