Transmission electron microscopy (TEM) measurements were carried out to investigate the spontaneously ordered structure in InxAl1-xAs epitaxial layers grown on (001) InP substrates. The selected area diffraction pattern showed two sets of superstructure reflections with symmetrical intensity at (100) and (010) positions, indicating that CuAu-I-type ordered structures with two different variants were formed in the InxAl1-xAs epitaxial layers. The dark-field TEM image showed that the size of the CuAu-I-type ordered domains with a needle-like shape was approximately 3∼4 nm thick, with lengths ranging from 10 to 20 nm. Based on the TEM results, explanations are given to describe the formation of only two variants of CuAu-I-type ordering. © 2003 American Institute of Physics.