Using an ultra-high-vacuum (UHV) scanning tunneling microscope (STM), we have investigated surface atomic structures of single-crystalline (1 0 0) diamond homoepitaxially grown by means of a microwave-plasma (MWP) chemical-vapor-deposition (CVD) method. STM images taken from B-doped (p-type) as-grown (H-terminated) diamond samples showed partially amorphous-like structures and partially unclear features related to (2 x 1)/(1 x 2) structures characteristic of H-terminated diamond (1 0 0). While the latter structures became clearer with increasing periods and temperatures of UHV annealing treatments well below 500 degreesC, substantially clear (2 x 1)/(1 x 2) structure images were obtained only after a sufficient annealing above 500 degreesC. A moderate UHV annealing at 150-200 degreesC sometimes resulted in the presence of disordered short-range (3 x 1) structures featured by row separations of 0.35 and 0.40 nm in very limited areas beside the (2 x 1)/(1 x 2) structures. These observations suggest the existence of both C-H and C-H, bonds at the topmost surfaces, substantially small but possible changes in surface fractions of the C-H and C-H, species during the UHV annealing and the final surface occupation of the C-H bonds. In addition, STM images differently featured for both occupied and unoccupied states of the (2 x 1) structure are discussed in relation to bonding and anti-bonding states of the surface C-H bonds. (C) 2003 Elsevier Science B.V. All rights reserved.