Abstracr:We report the results of the synthesis of IrO2 nanocrystals (NCs) on sapphire (SA) substrates with different orientations via metal-organic chemical vapor deposition (MOCVD) using (MeCp)(COD)Ir as the source reagent. The surface morphology and structural properties of the as-deposited NCs were characterized using field emission scanning electron microscopy (FESEM), and X-ray diffractometry (XRD). FESEM micrographs reveal that vertically aligned NCs were grown on SA(1 0 0), the NCs on SA(0 1 2) and SA(1 1 0) contained, respectively, single- and double-aligned directions with a tilt angle of 35° from the normal to the substrates, while NCs parallel in-plane alignment were gown on SA(0 0 1). The XRD results indicate that the IrO2 NCs are (0 0 1), (1 0 1), and (1 0 0) oriented on SA(1 0 0), SA(0 1 2)/SA(1 1 0) and SA(0 0 1) substrates, respectively. A strong substrate effect on the alignment of the IrO2 NCs growth has been demonstrated and the probable mechanism for the formation of the NCs is discussed.