Fe/MgO/FeCo epitaxial magnetic tunnel junctions (MTJs) were prepared on MgO(100) single crystal substrates by using in situ plasma oxidation for the formation of MgO barriers. The epitaxial relationship of Fe(001)/MgO(001)/FeCo(001) and Fe[100]//MgO[110]//FeCo[100] in the junctions was observed by reflection high-energy electron diffraction. Tunneling transport was clearly observed at low temperatures below about 150 K, and the barrier height of MgO is estimated to be 0.9 eV, which is smaller than the value expected from half of the band gap of bulk MgO. Tunnel magnetoresistance of 23% and 20% was observed at 4.2 and 77 K, respectively. The results suggest that plasma oxidation is useful for fabricating epitaxial magnetic tunnel junctions.