The miniaturization of Si-based devices requires control of doping profile, which makes the understanding of dopant interaction and diffusion in Si critical. The authors have studied the effect of As doping on P diffusion in Si using first-principles calculations. The authors found a form of As-vacancy complex is energetically favorable, allowing As to consume the vacancy so as to prohibit the vacancy-mediated P diffusion. Also, in the vicinity of As, the vacancy-mediated P diffusion barrier is increased, decreasing further the P mobility. The results provide useful guidance for designing As-doped barriers to block P diffusion in Si wafer processing and metal oxide semiconductor field-effect transistor device fabrication.