Pit formation and surface morphological evolution in Si(001) homoepitaxy are investigated by using scanning tunneling microscopy. Anti-phase boundary is found to give rise to initial generation of pits bound by bunched D B steps. The terraces break up and are reduced to a critical nucleus size with pit formation. Due to anisotropic kinetics, a downhill bias diffusion current, which is larger along the dimer rows through the centre area of the terrace than through the area close to the edge, leads to the prevalence of pits bound by {101} facets. Subsequent annealing results in a shape transition from {101}-faceted pits to multi-faceted pits. Pit formation and surface morphological evolution in Si (001) homoepitaxy are investigated by using scanning tunneling microscopy. Anti-phase boundary is found to give rise to initial generation of pits bound by bunched DB steps. The terraces break up and are reduced to a critical Due to anisotropic kinetics, a downhill bias diffusion current, which is larger along the dimer rows through the center area of ​​the terrace than through the area close to the edge, leads to the prevalence of pits bound by {101 } facets. Subsequent annealing results in a shape transition from {101} -faceted pits to multi-faceted pits.