The effect of rapid thermal annealing (RTA) on the electrical properties of heavily boron‐doped silicon epilayer grown at 680 °C by molecular‐beam epitaxy and coevaporation of B2O3 is studied. Through the RTA process, the boron clusters in the epilayer break into boron atoms and the interstitial boron enters the substitutional site. These two effects cause the improvement of the electrical properties of the silicon epilayer. After RTA at 1100 °C for 10 s, the hole concentration can reach 3.1×1020 cm-3 with the mobility of 39 cm2/V s, which is about the same as that of the bulk silicon, while the oxygen concentration is less than 1018 cm-3. It is also shown that the full width at half‐maximum of the x‐ray‐diffraction rocking curve decreases as the RTA temperature increases. The RTA process does not affect the steep distribution of carrier concentration at the epilayer/substrate interface which differs by about 6 orders of magnitude across the interface with the leading edge slope of 25–30 nm/decade.