We study B diffusion in the presence of Ge by using the first principles density functional theory calculation. We investigate the relative stability and migration barriers of Si and Ge interstitials as well as binding energy and the diffusion pathway of Boron-Interstitial (BI) pair comprised of Boron and Si or Ge interstitials. We find that Ge interstitials are more stable but less mobile compared to Si interstitials, leading to higher population of interstitials in the implanted Si 1-xGe x. However, BI pair comprised of the Ge interstitial and Boron is less stable compared to the Si interstitial -Boron pair. Also, the migration barrier of the BI pair in the presence of Ge is increased, leading to less TED.