We investigated boron (B) activation and diffusion in Si following millisecond annealing in the nonmelting regime by infrared laser. Simulations of the thermal field were also performed to calculate the process temperature. The laser annealed (LA) samples show an electrical activation higher by a factor of 2 as compared to rapid thermal annealed (RTA) samples, for comparable diffusion. When LA samples are further processed by RTA, the final diffusion extent is almost identical to an RTA treated sample, while electrical activation occurs. We argue that B atoms can temporarily occupy substitutional sites during the early stage of annealing; further annealing leads to interstitial clustering and dissolution, which enhance dopant diffusion during rapid thermal processing.