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. This paper reports bonding mechanism investigated from the results of ex-situ and in-situ measurements. As an ex-situ measurement, we firstly investigate effect of the application of ultrasonic vibration on magnitude of plastic deformation of the compliant bump. We show that "softening" of the bumps take place under the application of ultrasonic vibration. Second, change in crystal texture near the bonded interface was analyzed to clarify how the ultrasonic bonding produce bonded interface at room-temperature. As an in-situ measurement, dynamic strain generated under the bonding pad is measured by using a strain gauge made of Si.