CuAu‐I type ordered structures in InGaAs grown on (110)InP substrates by molecular beam epitaxy, have been studied by transmission electron microscopy. In the electron diffraction pattern from the InGaAs, superstructure spots associated with CuAu‐I type ordered structure are found. The intensity of the superstructure spots becomes stronger as the tilting angle of the substrate increases up to 5°. In high‐resolution images of the crystal, doubling in periodicity of 220 and 200 lattice fringes is found, which is associated with CuAu‐I type ordered structure. Moreover, anti‐phase boundaries are very often observed in the ordered regions, which has been suggested by Kuanetal. [Appl. Phys. Lett.51, 51 (1987)]. It is also found that ordering is not perfect, and that ordered regions are plate‐like microdomains lying on planes slightly tilted from the (110) plane. From these results, it is suggested that atomic steps on the growth surface play an important role in the generation of ordered structures.