For electrical spin injection and detection of spin-polarized electrons in silicon, we explore highly epitaxial growth of ferromagnetic full-Heusler-alloy Co2FeSi on silicon substrates using low-temperature molecular beam epitaxy (LT-MBE). Although in-situ reflection high energy electron diffraction images clearly show two-dimensional epitaxial growth for growth temperatures (T-G) of 60, 130, and 200 degrees C, cross-sectional transmission electron microscopy experiments reveal that there are single-crystal phases other than Heusler alloys near the interface between Co2FeSi and Si for T-G; = 130 and 200 degrees C. On the other hand, almost perfect heterointerfaces are achieved for T-G = 60 degrees C. These results and magnetic measurements indicate that highly epitaxial growth of Co2FeSi thin films on Si is demonstrated only for T-G = 60 degrees C. (C) 2009 Elsevier B.V. All rights reserved.