We investigate B diffusion in strained Si by using first-principles density functional theory calculations. An enhancement and an anisotropy of B diffusion in biaxial tensile strained Si are found. The diffusion barrier along the strain plane (channel) is decreased while the barrier in the vertical direction (depth) remains unchanged. This anisotropy comes from the orientation dependence of the saddle point in the diffusion pathway. The formation enthalpy of B-I pair also decreases in strained Si. According to our calculations, for strained Si on a Si 0.8Ge 0.2 buffer layer, which is widely used in strained metal oxide semiconductor field-effect-transistor, an enhancement of B diffusivity along the channel by a factor ∼4 and a factor ∼ 2 in the vertical direction are expected for typical rapid thermal anneals.