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 3similar to4 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. (C) 2003 American Institute of Physics.