We study the self-diffusion mechanism in silicon at low and high temperatures using the first-principles self-consistent pseudopotential method. We examine the stabilities of various self-interstitial configurations in silicon, and calculate the migration barriers for three different migration paths. We find that the Bourgoin-Corbett athermal migration is possible at low temperatures under high energy electron radiation. At high temperatures, thermal migration along the path from the [110] split site to the hexagonal site is dominant. The corresponding migration barrier is 0.31 eV, and the activation energy is 4.46 eV.