Giant tunnel magnetoresistance (TMR) ratios of up to 1995% at 4.2K and up to 354% at 290K were obtained for epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions (MTJs) featuring a reduced lattice mismatch in the MTJ trilayer by introducing a thin Co2MnSi lower electrode deposited on a Co50Fe50 buffer layer. The obtained giant TMR ratios can be explained by the enhanced contribution of coherent tunneling originating from the increased misfit dislocation spacing at the lower and upper interfaces with a MgO barrier along with the half-metallicity of Co2MnSi electrodes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4755773]