The three-dimensional (3D) island structure was prepared by molecular beam epitaxy for the lattice mismatchedInAs/GaAs(001)system and its images showing atomic structure on faceted planes were takenin situby ultrahigh vacuum scanning tunneling microscopy (STM). The (113), (114), and (215) faceted planes are observed for the 3D islands. Based on the STM images, atomic structural models are proposed for the faceted surfaces. The surface structure of the (113) faceted planes we propose is different from those observed on the flatGaAs(113)surface. ©1998 American Institute of Physics.