Abstract : This work aims at studying various DNA molecules with an atomic force microscope in the Tapping mode. DNA mechanical properties and its interaction with the surface at the nanometer scale are investigated. The microscope is sensitive to the interaction forces between a nanotip and an object or a surface. In the Tapping mode, the tip oscillates close to the surface at a drive frequency of about a few hundred kilohertz and with amplitudes of a few ten nanometers. The dynamic behavior of the oscillator may become non-linear depending on the strength of the tip-surface interaction and the oscillations conditions. Varying the drive frequency or the drive amplitude gives the opportunity to control the strength of the tip-surface interaction. Thus it's shown that a single DNA molecule may be imaged using various interaction modes : a pure attractive one, a pure repulsive one and a mixing of the two. In addition, it is shown that to extract quantitative information from DNA images requires to control the size and pollution of the tip, but also the quality of the surfaces on which the DNA is deposited. Following that way, silica surfaces were grafted with silanes molecules. Three different kinds of DNA molecules were studied. A long chain of 2500 non periodic bases pairs (bp), a shorter chain of 450bp made of Adenine bases on a single strand and of Thymine ones on the complementary strand and a chain with the same length, but made of alternated Adenine and Thymine bases on each strand. The experimental use of various regimes provides geometrical, mechanical and physicochemistry information about the conformations of the chains.