Single-crystal hexagonal perovskite YAlO3 has been attained through postdeposition rapid thermal annealing with temperatures above 900 °C on nanolaminated atomic-layer-deposited Y2O3 (2.03 nm)/Al2O3 (1.08 nm) multilayers. The perovskite film is epitaxially grown on GaAs(111)A substrates. The crystallography of the heterostructure was studied utilizing synchrotron radiation x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The epitaxial relationship between YAlO3 and GaAs is YAlO3 ( 0001 ) [ 11 2 ¯ 0 ] ∥ GaAs ( 111 ) [ 10 1 ¯ ], as determined from the radial scan along the in-plane direction. The cross-sectional STEM image reveals that the crystalline YAlO3 is continuous and the XRD study detects no other crystalline phases.