The doping reaction of P in the P-doped Si growth using Si2H6 and PH3 has been studied by the observation of the surface P during the growth with thermal desorption spectroscopy. When the growth temperature is set at 700 degrees C, the surface P chemistry is strongly dependent upon the PH3 concentration in the Si2H6 source. In the lower concentration below 50 ppm, the adsorbed P atoms form Si-P heterodimers, while P adsorbs on the defect site in the higher concentration region. The surface P-coverage has a linear relation with the doping concentration, which can be calculated by the two-site exchange model. The reaction scheme of the P-doped Si growth is discussed in the present paper.