The growth temperature dependence of the interface perfection of full Heusler alloy Co2FeSi/GaAs(001) hybrid structures have been examined using transmission electron microscopy (TEM) and high-resolution x-ray diffraction (HRXRD). The film grown at 100 °C shows an atomically abrupt interface without interfacial reaction. In the high-resolution transmission electron microscope (HRTEM) image from the 200 °C film, however, a 1–2 ML (monolayer) interlayer having a contrast different from both Co2FeSi and GaAs was observed at the interface, indicating that interfacial reaction starts at this growth temperature. The layer grown at 350 °C shows a further reacted interface in the HRTEM image, i.e., undulations and large steps at the interface. The interface perfection strongly correlates with the in-plane uniaxial magnetic anisotropy (UMA) as the UMA constant rapidly decreases above 200 °C in accordance with the progress of the interfacial reaction.