The influence of the growth temperature on the atomic ordering and interlayer diffusion of Heusler alloy Co2FeSi films grown on GaAs(001) substrates has been studied using high-resolution transmission electron microscopy. The Co2FeSi/GaAs(001) films grown below 200 degrees C show a coexistence of the Heusler-type L2(1) phase and the disordered B2 phase, which can be seen mainly near the interface, due to the low growth temperature. This phase coexistence can affect the spin polarization of the Co2FeSi layer near the interface. On the other hand, the film grown at an elevated temperature of 300 degrees C shows a uniformly atomically ordered L2(1) phase, indicating that 300 degrees C is approximately the transition temperature to the atomically ordered L2(1) structure. The elevation of the growth temperature, however, results in the formations of interfacial compounds above 250 degrees C, which could be detrimental to the efficient electrical spin injection. (C) 2007 American Vacuum Society.