Double-stranded (ds) DNA was imaged by noncontact-mode atomic force microscopy in an ultrahigh vacuum (UHV) after thermal annealing (<100 degrees C). The contour length of DNA measured (3300 Angstrom) was consistent with that of the B-form DNA (1000 bp) we prepared. In addition, we resolved right-handed helical turns with a spacing of 33 +/- 2 Angstrom. These observations showed both UHV condition and the thermal annealing did not make DNA take A-form but kept B-form.