The colloidal solutions of nanoparticles have been seen as promising solutions forheat transfer enhancement. Additionally, there has been an accelerated study on the effects of ultrasound on heat transfer enhancement in recent years. A few authors have studied the combined impact of Al2O3 nanofluids and ultrasound on mini channels. This study focused on the combined effects of Al2O3 nanofluids and ultrasound on heat transfer enhancement in a circular mini channel heat sink. Two concentrations of Al2O3-water nanofluids, i.e., 0.5% and 1%, were used for the experiments in addition to two heat input conditions, namely 40 W and 50 W providing a constant heat flux of 25000 W m-2 and 31250 W m-2 respectively. The effect on the nanofluids using 5 W ultrasound was analyzed. Experimental observations show that the usage of ultrasound increased the heat transfer coefficient. The heat transfer coefficient also increased with increasing nanoparticle concentration and high heat flux. The average heat transfer coefficient enhancement for 0.5% and 1% nanofluid due to increased heat flux in the absence of ultrasound was 12.4% and 9% respectively. At a constant heat input of 40 W, the induction of ultrasound enhanced the heat transfer coefficient by 22.8% and 23.9% for 0.5% and 1% nanofluid respectively. Similarly, for a constant heat input of 50 W, the usage of ultrasound enhanced the heat transfer coefficient by 19.8% and 22.9% for 0.5% and 1% nanofluid respectively Also, interesting findings are reported with low heat input with ultrasound vs. high heat input without ultrasound (i.e., 40 W with US vs. 50 W without US). The heat transfer coefficient and Nusselt number for 0.5% and 1% concentrations was enhanced by 9.2% and 13.6%, respectively. Furthermore, for fixed heat input powers of 40 W and 50 W, increasing the concentration from 0.5% to 1% along with ultrasound yielded an average enhancement in Nu of 38.3% and 32.4% respectively