The effect of rapid thermal annealing on the electrical properties of heavily boron doped Si epilayer grown by molecular beam epitaxy and coevaporation of B２Ｏ3 is studied. It is found that an increment of carrier concentration by a factor of 4 and an improvement of the Hall mobility equivalent to that of bulk Si at the same doping concentration are achieved by annealing at 1100℃ for 10s. The rapid thermal annealing process does not affect the steep distribution of carrier concentration at the epilayer/substrate interface which differs about 6 order of magnitude across the interface with the leading edge slope of 25-30nm /decade.