We fabricated single-crystal Fe/MgO/Fe magnetic tunnel junctions (MTJs) and achieved a magnetoresistance (MR) ratio of 88% at T = 293K (146% at T = 20K), the highest value yet reported. This MR ratio exceeds the theoretical limit for the conventional MTJs with an aluminum-oxide tunnel barrier. The bias-voltage dependence of the MR was very small, resulting in a high output voltage of 380 mV. This high voltage will help overcome problems in the development of next-generation ultrahigh-density MRAM.