Plasmid pBR322 DNA (0.5 mg/ml) isolated from Eschericha coli HB101 was suspended in Tris-HC1/EDTA (1/0.1 mol/L, pH 8.5), deposited on a highly oriented pyrolytic graphite surface, then imaged with scanning tunneling microscope (STM) in air and under paraffin liquid in constant current mode. The methods for preparing DNA samples used in the STM study are discussed. Dynamic changes of apparent width of supercoiled pBR322 DNA were captured during an STM imaging course. The pBR322 DNA is also visualized with an atomic force microscope (AFM) built by this laboratory after the sample was absorbed on the freshly cleaved mica surface. Characteristically relaxed circular structures of pBR322 DNA have been observed in the AFM experiment.