A CoSi2 layer was grown in-situ on heavily arsenic-doped Si by reactive chemical vapor deposition of a Co(η5-C5H5)(CO)2 precursor at 650℃. The nucleation and growth mechanism were investigated in comparison with those on undoped Si. In the initial deposition stage, discrete CoSi2 plates with a large area of the {111} coherent planes were nucleated with a deeper penetration depth and a higher density of twinned structure compared to the plates on undoped Si. A thicker CoSi2 layer is necessary for an epitaxial layer with uniform thickness on the heavily arsenic-doped Si. Analyses of the X-ray rocking curve and residual stress indicated that the high As concentration in CoSi2 reduced the lattice mismatch between Si and CoSi2 and reduced the lattice strain.