Effects of extended defect evolution on the anomalous diffusion of ion‐implanted boron during rapid thermal annealing (RTA) have been studied using transmission electron microscopy and secondary‐ion mass spectroscopy. It has been found that for low‐dose boron implants (<1×1014cm−2), no extended defects can be observed after RTA at 1000 °C, and the anomalous diffusion saturates within less than 10 s. However, extended defects are developed for high‐dose boron implants (>5×1014cm−2), and the anomalous diffusion persists for a much longer time and is dose dependent. Extended defect evolution has been characterized and correlated with the observed anomalous boron diffusion behaviors.