The present study reports the fabrication of Al-4.4Cu/2TiB2 nanocomposite from its microcomposite via post in-situ reaction ultrasonic treatment (UT). For the purpose of accomplishing post in-situ reaction ultrasonic treatment, the in-situ formed Al-4.4Cu/2TiB2 microcomposite was remelted and subjected to high-intensity ultrasonic waves for various length of time. Post in-situ reaction ultrasonic treatment of Al-4.4Cu/2TiB2 composite has resulted significant reduction in the size of TiB2 particles down to nano range. Ultrasonic cavitation aided particle dissolution and re-precipitation has been proposed as a mechanism responsible for the substantial reduction of TiB2 particle size. The compressive yield strength of Al-4.4Cu/2TiB2 nanocomposite formed via formed post in-situ reaction ultrasonic treatment is approximately two times higher than that of Al-4.4Cu alloy. The improved mechanical property of nanocomposite has been attributed to the presence of TiB2 particles of nanometer size and robust interfaces with the aluminum alloy. © 2016 Elsevier B.V.