We have investigated atomic structures of B-doped microwave-plasma chemical vapor-deposited diamond (10 0) surfaces using an ultra-high-vacuum scanning tunneling microscope (STM). Most of STM images taken from the diamond surfaces at the very initial oxidization stage reflected the original surface atomic regularity of the diamond (1 0 0):H 2 x 1/1 x 2 structures while some of these images demonstrated a distinctive feature clearly differing from those of the original hydrogen-terminated surfaces. A further oxidization of the diamond surfaces resulted in the appearance of another specific feature in larger surface areas. Possible oxygen sites and oxidization process of hydrogen-terminated diamond (1 0 0) surfaces are discussed in relation to preferential oxidizations of C-C dimer bonds, based upon our STM observations. (c) 2004 Published by Elsevier B.V.