Structural properties of single crystalline CoGa grown by molecular beam epitaxy on a GaAs substrate were investigated using transmission electron microscopy. The results showed that the crystal orientation relationships between the CoGa epilayer and the GaAs substrate were [110BAR]CoGa\\[110BAR]GaAs and (001)CoGa\\(110)GaAs and that the CoGa/GaAs heterointerface had abruptness. Modulation of either the Co or the Ga was observed in the CoGa epitaxial layer.