We discuss bonding mechanism of ultrasonic bonding of cone-shaped bump. Room-temperature microjoining of Au-Au or Cu-Cu bumps in the air ambient has been achieved by using the cone-shaped bumps with ultrasonic assist. We have investigated two characteristics of ultrasonic bonding. We first investigate effect of the application of ultrasonic vibration on magnitude of plastic deformation of the compliant bump. We show that 'softening' of the bump takes place under the application of ultrasonic vibration. Secondly, change in crystal texture near the bonded interface was analyzed to clarify how the ultrasonic bonding produce bonded interface at room-temperature. Under application of ultrasonic vibration, recrystallization of grains takes place near the interface to transform to fine crystallites. The thermocompression bonding, on the other hand, generates fine crystals in the bulk of the cone-shaped bump. This difference in location where recrystallization generates can be interpreted by taking shear strain distribution into consideration. The room temperature bonding can be interpreted by the generation of fine crystallites at the interface which results in breaking of a contaminant layer at the interface.