Single-molecule techniques can observe the individual interaction which has be buried by conventional biochemical techniques. Therefore, single-molecule methods offer valuable complementary information on detailed molecular mechanisms of enzyme functions1-8. There are a lot of breakthroughs in medical, physics, chemistry, biology or biochemistry because of these fast developments of single molecule techniques. For scientists in biochemistry or biophysics, the most popular and well-developed single molecule techniques are bead manipulation methods (such as optical tweezers, magnetic tweezers and tethered particle motion) and fluorescence based techniques (such as fluorescence resonance energy transfer (FRET). In this paper, we will introduce several single-molecule approaches to investigate "DNA metabolism", including homologous recombination, site-specific recombination, DNA package and nucleosome remodeling process and describe the obtained information.