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We present the systematic study of the growth mechanism of ZnO nanorods grown on Al 2 O 3 substrates with ZnO homo-buffer, n-GaN and p-GaN interlayers. Vertically aligned ZnO nanorods with diameter of 50 nm and lengths of range of 0.1 -2 µm were synthesized at the substrate temperature of 350 -500 o C by catalyst-free metal-organic chemical vapor deposition. A thin ZnO film was observed underneath the ZnO nanorods grown on Al 2 O 3 , ZnO homo-buffer layers and p-GaN interlayers and the film thickness varied from 0.1 µm to 0.55 µm depending on the substrates while the film was negligible on an n-GaN interlayer. The residual strain of the ZnO nanorods grown on Al 2 O 3 substrates was reduced to less than one fifth of the original value by employing a n-GaN interlayer. The n-GaN interlayer also enhanced the orientations among the nanorods in the ab-plane. The extended x-ray absorption fine structure measurements revealed that there were a substantial amount of disorders in the bonding lengths of the Zn-O pairs in the beginning of the ZnO nanorod growth on the Al 2 O 3 substrates with and without the n-GaN interlayer. The disorders of the Zn-O pairs located near the ab-plane were decreased as the nanorod length was increased above 0.1 µm implying that the strain relaxation of the ZnO crystals in the ab-plane was critical in the formation of ZnO nanorods.