In this paper, elastic moduli of nanostructured alumina are evaluated by simultaneous measurement of longitudinal and shear wave velocities using mode-converted ultrasound in scanning acoustic microscope (SAM). Mode-converted longitudinal and shear waves inside alumina sample are captured to calculate acoustic wave velocities and determine elastic constants such as Young’s modulus and Bulk modulus. Al2O3 nanostructured alumina samples are formed by compacting micro-sized Al2O3 powder with nano-sized Al2O3 powder from 10wt% to 50wt%, and tested by SAM to investigate elastic moduli. A correlation is found from experiment that the more percentage of nano-particles are added, the higher elastic moduli are obtained. It is also shown that the mode-converted ultrasound is sensitive enough to characterize mechanical modulus of nanostructured alumina quantitatively.