The most stable structures for the adsorption and dissociation of phosphine (PH3) on SiGe(100) (2 x 1) surface have been investigated by relative total energy calculations based on density functional theory. According to the optimization calculations, PH3 is adsorbed on the Si (down) and Ge (down) site of the Ge-Si and Ge-Ge dimers on SiGe surface, respectively. The PH2 and H products have been found to be thermodynamically favored in the dissociation path of PH3 on SiGe surface when the system is thermally activated. Although PH3 is adsorbed on the Ge-Ge and Ge-Si dimers directly, it dissociates on the SiGe surface by passing through a transition state. The asymmetric Ge-Si and Ge-Ge dimers on SiGe surface are found to be approximately symmetric after the dissociation of PH3 on the surface. The present work has showed that PH2 prefers to be adsorbed on Ge site of the Ge-Si dimer. Therefore, the adsorption of PH2 on Ge site of the Ge-Si dimer, while PH3 being dissociated on the Si site, has indicated the migration of PH2 on SiGe surface.