A transient-enhanced diffusion has been observed during the furnace or rapid thermal annealing of As-implanted Si. We have studied the relations of the enhanced diffusion to residual defects and lattice restoration in detail. The As concentration profiles and residual defects are measured. It is found from the data that the lattice has been restored when the implanted sample is annealed at 1150℃ (or 1050℃) for 1s. The defeet density decreases rapidly with increase of annealing time (from 1 to 12s). The enhanced diffusion coefficient maximum appears in the annealing time ranging from 1 to 5s. Almost a "complete" annealing of displacement damage is obtained and the diffusion coefficient is less than that in above-mentioned conditions when the implanted samples are annealed at 1150℃in the time ranging from 12 to 20s. The mechanism of lattice restoration and enhanced diffusion in annealing process have been discussed.