Low-temperature scanning tunneling microscopy (STM) was used to study the Bi-O plane of Bi2Sr2CuO6+delta (Bi-2201) single crystals cleaved in situ. The atomic-resolution STM images clearly show a commensurate modulated lattice structure. In contrast to the previous STM data obtained at room temperature, the missing atom rows" have been clearly seen at the ridges of the modulation. By comparing the low-temperature STM images of Bi-2201 and Bi-2212, we found that the modulation of the Bi-O plane in these two materials originates from the missing atom rows. The atomic steps at the surface have also been investigated at room temperature confirming that most easily exposed atomic layers in Bi-2201 are Bi-O and Sr-O planes, as in Bi-2212.